USMLE Step 2 CK neurology questions

The patient presents within 3 to 4.5 hours of clearly defined symptom onset, meets the extended-window criteria for IV alteplase, and has no exclusion criteria (no hemorrhage on CT, normal INR, normal platelets, no recent surgery). Current AHA/ASA guidelines support thrombolysis in this extended window for eligible patients, including those with NIHSS scores indicating moderate deficit. Noncontrast CT excluding hemorrhage is sufficient before treatment; MRI is not required and would delay therapy. Time to treatment is the single strongest predictor of good outcome, so alteplase should be given as soon as eligibility is confirmed.

• A. MRI is not necessary before thrombolysis and would only delay time-sensitive treatment.
• B. Aspirin alone is inferior to thrombolysis in a patient who still qualifies for alteplase.
• C. Thrombectomy is reserved for large vessel occlusion and does not replace IV alteplase when eligible.
• D. The patient is still within the eligible 4.5-hour extended window, so withholding treatment is incorrect.

In any patient with suspected acute stroke, noncontrast CT of the head is the first imaging study because it is fast, widely available, and reliably distinguishes ischemic stroke from intracerebral hemorrhage, which is essential before giving thrombolytics. MRI is more sensitive for early ischemic changes but takes longer and is not required to make the initial treatment decision. Vascular imaging and cardiac workup are useful afterward to determine stroke mechanism but should not delay the initial hemorrhage-exclusion step.

• B. CT angiography is useful for identifying large vessel occlusion but follows, rather than replaces, the initial noncontrast CT.
• C. Carotid ultrasound evaluates extracranial stenosis and has no role in the hyperacute hemorrhage-exclusion step.
• D. MRI is more time-consuming and not needed emergently to exclude hemorrhage before thrombolysis decisions.
• E. Echocardiography assesses for cardioembolic source and is performed later in the workup, not as the first imaging test.

This patient has a large vessel occlusion (proximal MCA) with a small infarct core and a large mismatch between the perfusion deficit and the infarct core, meeting criteria for mechanical thrombectomy up to 24 hours from last known well based on perfusion imaging criteria (DAWN/DEFUSE 3 trials). IV alteplase is not appropriate because he is well beyond the 4.5-hour thrombolytic window. Anticoagulation and dual antiplatelet therapy do not achieve rapid large-vessel recanalization. Hemicraniectomy is reserved for malignant edema with impending herniation, not as an initial reperfusion strategy.

• B. Heparin does not achieve mechanical recanalization and is not indicated in acute ischemic stroke.
• C. IV alteplase is only effective within 4.5 hours of last known well; this patient is at 10 hours.
• D. Hemicraniectomy is used for malignant cerebral edema with herniation, not as a primary reperfusion treatment.
• E. Dual antiplatelet therapy does not address a large vessel occlusion acutely.

In warfarin-associated intracranial hemorrhage, rapid reversal of anticoagulation is critical to limit hematoma expansion. Current guidelines recommend 4-factor prothrombin complex concentrate (PCC) for rapid factor replacement combined with intravenous vitamin K to sustain reversal, since PCC alone has a short duration of effect. Fresh frozen plasma corrects INR more slowly and requires larger volumes, increasing the risk of fluid overload, making it inferior to PCC. Vitamin K alone takes hours to normalize INR and is too slow for an acute hemorrhage. Protamine reverses heparin, and idarucizumab reverses dabigatran, neither of which is relevant to warfarin.

• A. Protamine sulfate reverses heparin, not warfarin.
• B. FFP works more slowly and requires larger infusion volumes compared with PCC, making it a second-line choice.
• D. Idarucizumab is a specific reversal agent for dabigatran, not warfarin.
• E. Vitamin K alone has a delayed onset of action and does not provide the rapid factor replacement needed in acute hemorrhage.

This presentation is a classic pure motor lacunar syndrome caused by a small infarct in the internal capsule. The underlying pathology in chronic, poorly controlled hypertension is lipohyalinosis, a degenerative process affecting small penetrating arteries that leads to vessel wall thickening, luminal narrowing, and eventual occlusion of these end arteries, producing small subcortical infarcts. There is no imaging or clinical evidence of carotid disease, a cardiac embolic source, vasculitis, or a PFO in this vignette, making those mechanisms less likely. Lacunar strokes typically present with pure motor, pure sensory, or ataxic-hemiparetic syndromes without cortical signs such as aphasia or neglect.

• A. Carotid atherosclerotic embolism typically produces cortical infarcts with cortical signs, and no significant carotid stenosis was found.
• B. PFO-related paradoxical embolism usually causes cortical or larger territory infarcts, not isolated lacunar lesions.
• C. Vasculitis typically involves medium-sized vessels and would present with other systemic or multifocal features, not an isolated classic lacunar syndrome.
• E. Cardioembolic strokes usually produce larger, cortical infarcts, and no left atrial dilation or arrhythmia is described.

For secondary prevention after a noncardioembolic ischemic stroke or TIA, long-term single antiplatelet therapy with aspirin or clopidogrel is the standard of care, as anticoagulation offers no additional benefit over antiplatelet therapy and carries a higher bleeding risk in this population. Aspirin monotherapy (or clopidogrel monotherapy) is appropriate here since there is no indication for dual antiplatelet therapy (which is reserved for minor stroke or high-risk TIA within the first 21-90 days) and no cardioembolic source requiring anticoagulation. Warfarin and direct oral anticoagulants are reserved for cardioembolic causes such as atrial fibrillation, which has been excluded here by monitoring and echocardiography. Heparin, whether unfractionated or low molecular weight, has no role in routine secondary prevention of noncardioembolic stroke.

• A. Apixaban is a direct oral anticoagulant used for cardioembolic stroke prevention, not noncardioembolic TIA.
• C. Warfarin is indicated for cardioembolic stroke sources such as atrial fibrillation, which has been excluded in this patient.
• D. Unfractionated heparin has no established role in secondary prevention of noncardioembolic ischemic stroke.
• E. Low molecular weight heparin is not indicated for long-term secondary stroke prevention in this setting.

In symptomatic patients with ipsilateral carotid stenosis of 70% or greater and low surgical risk, carotid endarterectomy performed within 2 weeks of the event provides a substantial reduction in subsequent stroke risk compared with medical therapy alone, as shown in large randomized trials. Medical therapy alone is insufficient for this degree of symptomatic stenosis and does not address the mechanical source of embolism. Carotid artery stenting is reserved for patients at high surgical risk for endarterectomy (such as significant cardiac disease or a hostile neck) rather than as first-line therapy in a low-risk patient. Anticoagulation has no proven benefit over antiplatelet therapy for atherosclerotic carotid stenosis, and delaying intervention with repeat imaging increases the risk of recurrent stroke.

• A. Stenting is generally reserved for patients who are poor candidates for surgery, not as the first-line choice in a low-risk patient.
• C. Delaying intervention with repeat imaging increases the risk of recurrent stroke; treatment should proceed promptly in a symptomatic, low-risk patient.
• D. Anticoagulation with warfarin has not been shown to be superior to antiplatelet therapy and is not standard for carotid atherosclerotic stenosis.
• E. Medical therapy alone is inferior to endarterectomy for symptomatic stenosis of 70% or greater in a low-risk surgical candidate.

Thunderclap headache (sudden, severe, peaking within seconds to minutes) raises strong suspicion for subarachnoid hemorrhage (SAH), most commonly from a ruptured saccular aneurysm. Noncontrast CT of the head is the best initial test and has very high sensitivity within the first 6 hours of symptom onset, but sensitivity declines afterward as blood is resorbed. When CT is negative but clinical suspicion remains high, lumbar puncture is the next step to look for xanthochromia (from red blood cell breakdown) and elevated opening pressure, which can detect SAH missed on CT. A negative CT alone is not sufficient to exclude SAH in a classic clinical presentation.

• A. Discharging a patient with thunderclap headache without excluding SAH risks missing a life-threatening aneurysmal bleed.
• B. Repeating CT in 24 hours delays diagnosis and risks a fatal rebleed if an aneurysm is present.
• D. MRI is less sensitive and practical than lumbar puncture for detecting subarachnoid blood after a negative CT in the acute setting.
• E. Migraine is a diagnosis of exclusion here; thunderclap onset mandates exclusion of SAH first.

This patient's presentation meets diagnostic criteria for migraine with aura: a fully reversible visual aura (positive features such as shimmering or zigzag lines) that develops gradually over 5-20 minutes and lasts less than 60 minutes, followed by a unilateral, throbbing headache with nausea and photophobia. The gradual spread of visual symptoms over minutes, rather than instantaneous onset, is characteristic of cortical spreading depression and helps distinguish migraine aura from vascular or seizure-related events. Recurrent stereotyped episodes with a normal interictal examination support a primary headache disorder rather than a structural or vascular cause.

• A. TIA typically causes sudden-onset negative neurologic symptoms (loss of function) rather than a gradually spreading positive visual phenomenon, and does not recur with the same stereotyped pattern over months.
• C. Cluster headache causes severe unilateral periorbital pain with autonomic features (lacrimation, rhinorrhea) in rapid, repeated bouts, not a preceding gradual visual aura.
• D. Tension-type headache is typically bilateral, pressing, and not preceded by visual aura or associated with nausea and photophobia of this severity.
• E. Occipital seizures typically cause brief, stereotyped visual hallucinations lasting seconds and may be followed by other ictal phenomena, not a gradual 15-minute aura with migrainous headache.

For moderate to severe migraine attacks that do not respond adequately to NSAIDs, a triptan such as sumatriptan is first-line acute therapy in patients without contraindications such as coronary artery disease, uncontrolled hypertension, or stroke history. Triptans are selective serotonin 5-HT1B/1D receptor agonists that cause cranial vasoconstriction and inhibit trigeminal nociceptive neurotransmission, aborting the migraine attack. They are most effective when taken early in an attack. This patient has no cardiovascular risk factors that would contraindicate triptan use.

• A. Opioids are not recommended for migraine treatment because of risk of medication overuse headache and dependence, and are reserved for refractory cases.
• B. Verapamil is used for cluster headache prophylaxis, not acute migraine treatment.
• C. Ergotamine is less selective than triptans, has more side effects, and is not first-line given better-tolerated alternatives.
• D. Propranolol is used for migraine prophylaxis, not for treating an acute attack.

This patient's progression from episodic to near-daily headache in the setting of frequent triptan use (≥10 days/month for triptans) is characteristic of medication overuse headache, a complication of chronic overuse of acute headache medications. The first and essential step in management is withdrawal of the overused agent, which is required for the headache pattern to improve; preventive therapy can be added but will be less effective if overuse continues. Education about limiting acute medication use (triptans and opioids to ≤10 days/month, simple analgesics to ≤15 days/month) is central to prevention of recurrence. Neuroimaging is not indicated given the normal examination and absence of red flags.

• A. Imaging is not indicated because there are no red flag features suggesting a secondary cause.
• C. Increasing triptan dose will worsen medication overuse headache rather than improve it.
• D. Starting prophylaxis alone without stopping the overused agent is unlikely to break the cycle of medication overuse headache.
• E. Corticosteroids have no established role in routine management of medication overuse headache.

This patient has cluster headache, characterized by severe unilateral periorbital pain with ipsilateral autonomic features (lacrimation, ptosis, conjunctival injection), occurring in bouts, with restlessness during attacks (a distinguishing feature from migraine, where patients prefer to lie still). High-flow oxygen (100% oxygen at 12-15 L/min via face mask for 15-20 minutes) is a first-line abortive treatment for acute cluster attacks, along with subcutaneous or intranasal triptans. Verapamil is used for prophylaxis, not acute abortive treatment. Recognizing the autonomic features and behavioral pattern is key to distinguishing cluster headache from migraine and tension-type headache.

• A. Intravenous magnesium is used for migraine, not as first-line abortive therapy for cluster headache.
• B. Verapamil is the first-line agent for cluster headache prophylaxis, not acute abortive treatment.
• D. Valproic acid is a preventive agent for headache disorders, not an acute abortive treatment.
• E. Acetaminophen is not effective for the acute severe pain of cluster headache.

This patient has tension-type headache, characterized by bilateral, non-pulsatile, pressing or band-like pain of mild-to-moderate intensity that is not aggravated by routine physical activity and lacks migrainous features such as nausea, photophobia, or phonophobia. First-line treatment for episodic tension-type headache is a simple analgesic such as ibuprofen, acetaminophen, or aspirin. Triptans are specific to migraine and cluster headache and are not effective for tension-type headache. If headaches become frequent or chronic, amitriptyline is the preferred prophylactic agent.

• B. Corticosteroids have no role in routine tension-type headache management.
• C. Opioid-related agents like butorphanol are not appropriate first-line therapy and carry risk of overuse and dependence.
• D. Topiramate is a migraine prophylactic agent, not first-line therapy for episodic tension-type headache.
• E. Triptans are not effective for tension-type headache and are reserved for migraine and cluster headache.

This patient has giant cell (temporal) arteritis, suggested by age over 50, new headache, jaw claudication, temporal artery tenderness, systemic symptoms, and markedly elevated ESR and CRP. Because untreated giant cell arteritis carries a high risk of irreversible vision loss from ophthalmic artery involvement, high-dose glucocorticoids should be started immediately on clinical suspicion, without waiting for biopsy confirmation. Temporal artery biopsy should still be performed, ideally within 1-2 weeks of starting steroids, since the characteristic histologic changes persist for some time after treatment initiation. Delaying treatment to obtain biopsy or imaging risks permanent blindness.

• A. Aspirin alone and observation do not address the underlying vasculitis and risk of vision loss.
• B. Delaying steroid treatment for 2 weeks risks irreversible vision loss and is not appropriate.
• C. MRI is not required before treatment and would unnecessarily delay time-sensitive steroid therapy.
• D. Methotrexate is used as a steroid-sparing agent in refractory or relapsing disease, not as initial therapy.

This patient's presentation of headache, papilledema, and abducens nerve palsy (a false localizing sign from elevated intracranial pressure) in an obese young woman, with normal MRI and venography excluding a structural cause or venous thrombosis, is classic for idiopathic intracranial hypertension. After neuroimaging has excluded a mass lesion or venous sinus thrombosis, lumbar puncture with measurement of opening pressure (with the patient in the lateral decubitus position) is required to confirm elevated intracranial pressure (typically >25 cm H2O in adults) and to analyze cerebrospinal fluid composition, which is normal in this condition. Acetazolamide, which reduces cerebrospinal fluid production, is the first-line medical treatment, often combined with weight loss.

• A. Repeating MRI in 6 weeks delays diagnosis and treatment of a condition that threatens vision.
• B. Visual evoked potentials are not used to diagnose idiopathic intracranial hypertension.
• C. Temporal artery biopsy is used to diagnose giant cell arteritis, which does not fit this presentation.
• D. CT angiography is not needed once MRI venography has already excluded venous sinus thrombosis.

A sudden-onset, severe headache reaching maximum intensity within seconds to minutes ("thunderclap headache") is a red flag requiring urgent evaluation for subarachnoid hemorrhage. The most appropriate next step is noncontrast CT of the head, which has high sensitivity for subarachnoid hemorrhage within the first 6 hours of symptom onset; if CT is negative but suspicion remains, lumbar puncture should follow to assess for xanthochromia. Neck stiffness in this setting may reflect meningeal irritation from subarachnoid blood rather than infectious meningitis. Empiric symptomatic treatment or outpatient imaging would dangerously delay diagnosis of a life-threatening hemorrhage.

• A. Treating empirically without imaging risks missing a life-threatening subarachnoid hemorrhage.
• B. Outpatient imaging in a week is far too delayed for a thunderclap headache with possible hemorrhage.
• D. Discharging home without imaging is unsafe given red flag features suggestive of hemorrhage.
• E. There is no evidence supporting bacterial meningitis, and steroids are not appropriate without further workup.

This patient has status epilepticus, defined as a single seizure lasting more than 5 minutes or recurrent seizures without return to baseline between them. With intravenous access already established, the first-line treatment is an intravenous benzodiazepine, such as lorazepam or diazepam, which rapidly enhances GABAergic inhibition and terminates seizure activity in most patients. If seizures persist despite an adequate benzodiazepine dose, a second-line intravenous antiepileptic agent (fosphenytoin, valproate, or levetiracetam) should be given. Intramuscular midazolam is preferred over intravenous benzodiazepines only when IV access is not available, which is not the case here.

• A. Intubation and general anesthesia are reserved for refractory status epilepticus unresponsive to first- and second-line agents.
• B. Phenytoin (or fosphenytoin) is a second-line agent used if seizures continue despite an adequate dose of benzodiazepine.
• C. Levetiracetam is a second-line agent, not first-line, for status epilepticus.
• E. Intramuscular midazolam is reserved for situations without IV access; IV access is already established in this patient.

Status epilepticus unresponsive to two adequate doses of a benzodiazepine is considered benzodiazepine-refractory and requires a second-line intravenous antiepileptic, most commonly fosphenytoin, valproic acid, or levetiracetam. Fosphenytoin is water-soluble and can be infused faster than phenytoin with less risk of injection-site necrosis, making it a standard second-line choice. Repeating benzodiazepines beyond two doses increases the risk of respiratory depression and hypotension without added seizure control. Third-line agents such as propofol or phenobarbital for general anesthesia are reserved for true refractory status epilepticus that fails second-line agents.

• A. A third benzodiazepine dose risks respiratory depression without additional benefit once two doses have failed.
• B. General anesthesia is reserved for refractory status epilepticus that fails second-line therapy, not as an immediate next step.
• D. Phenobarbital is not the preferred first second-line agent given its sedating and respiratory-depressant profile compared with fosphenytoin or levetiracetam.
• E. Intramuscular midazolam is used as a first-line agent when IV access is unavailable, not indicated here since IV access is already established.

This child's brief staring spells without aura or postictal confusion, provoked by hyperventilation, with generalized 3-Hz spike-and-wave discharges on EEG, are characteristic of childhood absence seizures. Ethosuximide is first-line therapy, working by blocking T-type calcium channels in thalamic neurons that generate the 3-Hz rhythm. Valproic acid is an alternative first-line agent, particularly if generalized tonic-clonic seizures coexist. Carbamazepine and phenytoin can worsen absence seizures and are contraindicated in this condition.

• A. Carbamazepine can exacerbate absence seizures and is not used for this seizure type.
• B. Phenytoin is ineffective for absence seizures and can worsen them.
• D. Levetiracetam is not first-line for absence seizures, though sometimes used as adjunct therapy.
• E. Lamotrigine is a reasonable alternative but is not the first-line agent; it is used when ethosuximide or valproate are not tolerated.

Levetiracetam is widely used as first-line therapy for focal-onset epilepsy, including focal seizures with secondary generalization, due to its favorable efficacy, tolerability, and low drug-interaction profile compared with older agents. Carbamazepine, lamotrigine, and oxcarbazepine are also acceptable first-line options for focal epilepsy. Ethosuximide is specific to absence seizures and has no role in focal epilepsy. Valproic acid is more often reserved for generalized epilepsies and has significant teratogenic and metabolic adverse effects that make it less favorable as a first choice here.

• A. Ethosuximide treats only absence seizures and is ineffective for focal epilepsy.
• B. Clonazepam is used for acute seizure termination or as adjunct therapy, not as first-line maintenance monotherapy for focal epilepsy.
• C. Valproic acid is preferentially used for generalized epilepsies and carries significant teratogenic risk, making it a less ideal first choice.
• E. Phenobarbital has a poor adverse effect and sedation profile, making it a second- or third-line agent today.

Nystagmus, ataxia, and gingival hyperplasia in a patient on phenytoin with a supratherapeutic level (32 mcg/mL) are classic signs of phenytoin toxicity. Gingival hyperplasia results from phenytoin-induced fibroblast proliferation in gum tissue and occurs with chronic use independent of acute toxicity, while nystagmus and ataxia are dose-dependent cerebellar-vestibular toxic effects that worsen as levels rise above the therapeutic range. Management involves dose reduction and monitoring levels. The other options describe adverse effects of different antiepileptics, not features seen in this patient.

• A. Valproic acid hepatotoxicity presents with jaundice, elevated liver enzymes, and malaise, not nystagmus or gum overgrowth.
• B. Stevens-Johnson syndrome from lamotrigine presents with mucocutaneous blistering rash, not ataxia or gingival hyperplasia.
• C. Aplastic anemia from carbamazepine presents with pancytopenia and infection/bleeding, not ataxia and gingival hyperplasia.
• E. Levetiracetam can cause irritability or depression but does not cause nystagmus, ataxia, or gingival hyperplasia.

Valproic acid is associated with the highest risk of major congenital malformations (including neural tube defects) and long-term neurodevelopmental impairment among antiepileptic drugs, so women of childbearing age should be counseled about transitioning to lower-risk alternatives such as lamotrigine or levetiracetam, ideally before conception is attempted. Abruptly stopping or switching medications without planning risks seizure recurrence, so this discussion should occur proactively given this patient's stated future pregnancy intentions. Folic acid supplementation is recommended for all patients with epilepsy of childbearing potential but does not mitigate valproate's teratogenic risk sufficiently to justify continuing it unchanged. Phenobarbital also carries teratogenic risk and is not preferred over levetiracetam or lamotrigine.

• A. Continuing valproic acid unchanged ignores her expressed future pregnancy plans and the drug's high teratogenic risk.
• B. Increasing the dose would increase fetal exposure and malformation risk, which is inappropriate.
• D. Folic acid is recommended but does not sufficiently reduce valproate's unique teratogenic risk; a regimen change should still be discussed.
• E. Phenobarbital also carries significant teratogenic risk and offers no advantage over lamotrigine or levetiracetam.

This patient has eclampsia, defined as new-onset seizures in a woman with preeclampsia (hypertension and proteinuria). Intravenous magnesium sulfate is the drug of choice for both treatment and prevention of eclamptic seizures, acting as an NMDA-receptor antagonist and cerebral vasodilator, and has been shown in randomized trials to be superior to phenytoin and benzodiazepines for this indication. Magnesium also reduces the risk of recurrent seizures and maternal mortality compared with these alternatives. Phenytoin and benzodiazepines are reserved for cases refractory to magnesium or when magnesium is contraindicated.

• B. Phenytoin is inferior to magnesium sulfate for eclamptic seizure prophylaxis and treatment based on randomized trial data.
• C. Oral phenobarbital is not appropriate for acute seizure prevention in eclampsia; it also would not act quickly enough in this acute setting.
• D. Levetiracetam has not been shown to be superior to magnesium sulfate and is not standard therapy for eclampsia.
• E. Diazepam is reserved for second-line or acute seizure termination if magnesium fails, not as primary therapy.

This child had a simple febrile seizure: generalized, lasting less than 15 minutes, with no recurrence within 24 hours, occurring in a neurologically normal child aged 6 months to 5 years with fever. Simple febrile seizures do not require neuroimaging, EEG, or lumbar puncture when the child returns to baseline and has no signs of meningitis or CNS infection. Lumbar puncture is reserved for infants with meningeal signs, prolonged altered mental status, or those under 6 months or with incomplete immunizations where clinical signs of meningitis may be masked. Daily prophylactic antiepileptic therapy is not indicated even for recurrent simple febrile seizures given the benign prognosis.

• B. Head CT is not indicated for a simple febrile seizure with a normal neurologic exam.
• C. EEG is not indicated for simple febrile seizures as it does not predict recurrence or future epilepsy.
• D. Lumbar puncture is not required in a well-appearing, fully immunized child with a simple febrile seizure and no meningeal signs.
• E. Prophylactic antiepileptic medication is not recommended for simple febrile seizures regardless of recurrence.

In suspected bacterial meningitis, empiric antibiotics and adjunctive dexamethasone should be started immediately after blood cultures are drawn and should not be delayed for neuroimaging or lumbar puncture, since delay increases mortality and neurologic morbidity. Empiric therapy in an immunocompetent adult covers the most likely pathogens (Streptococcus pneumoniae, Neisseria meningitidis) with vancomycin plus a third-generation cephalosporin such as ceftriaxone. Dexamethasone is given with or just before the first antibiotic dose because it reduces inflammation-mediated complications, particularly in pneumococcal meningitis, and is most effective when started early. Ampicillin is added separately only when Listeria is a concern, such as in patients over 50 with additional risk factors, but it does not replace vancomycin and ceftriaxone here as sole therapy.

• A. Ampicillin alone does not cover pneumococcus or meningococcus adequately and is insufficient empiric monotherapy.
• B. Withholding antibiotics for any CSF test results risks fatal delay in a time-sensitive emergency.
• C. Delaying dexamethasone until after CSF results reduces its benefit; it should be given with the first antibiotic dose.
• D. Narrowing to ceftriaxone alone based on blood culture results ignores the need for broad empiric coverage from the outset given resistant pneumococcal strains.

This CSF profile, markedly elevated opening pressure, neutrophil-predominant pleocytosis, high protein, and low CSF-to-serum glucose ratio (here 28%, well below the 0.4-0.5 cutoff), is classic for acute bacterial meningitis. Bacteria consume glucose and increase vascular permeability, producing hypoglycorrhachia and elevated protein, while the acute neutrophilic response drives the cell count up. Viral meningitis typically shows a lymphocytic pleocytosis with normal glucose and only mildly elevated protein. Tuberculous and fungal meningitis typically present more subacutely with lymphocytic predominance, markedly elevated protein, and low glucose, but the acute neutrophilic picture here argues against them.

• A. Fungal meningitis also usually shows lymphocytic predominance and a subacute presentation.
• B. Normal pressure hydrocephalus does not cause fever, elevated WBC count, or abnormal glucose/protein in CSF.
• C. TB meningitis typically has a lymphocytic predominance and a more subacute course over weeks.
• E. Viral meningitis shows lymphocytic predominance and normal glucose, not neutrophilic pleocytosis with hypoglycorrhachia.

Herpes simplex encephalitis classically presents with fever, altered mental status, focal neurologic deficits, and seizures, with MRI showing temporal and insular lobe involvement and EEG showing periodic lateralized epileptiform discharges. Because untreated HSV encephalitis has high mortality and morbidity, intravenous acyclovir should be started empirically as soon as the diagnosis is suspected, without waiting for PCR confirmation, since treatment delay worsens outcomes. CSF HSV PCR is the most accurate confirmatory test but takes time to process, and therapy is continued or stopped based on the result once available. Brain biopsy was historically used before PCR but is now reserved for atypical or PCR-negative cases.

• A. Oral valacyclovir has not been shown to be effective for encephalitis, and outpatient management is inappropriate for this severity of illness.
• B. Brain biopsy is invasive and reserved for cases where PCR is unavailable or inconclusive, not a prerequisite for treatment.
• C. The CSF and MRI findings are not typical of bacterial meningitis, and this would miss the correct diagnosis.
• E. Waiting for PCR confirmation before treating risks irreversible neurologic damage or death; treatment is empiric and urgent.

Close contacts of a patient with invasive meningococcal disease, including household members, roommates, and anyone with direct exposure to oral secretions, should receive chemoprophylaxis as soon as possible, ideally within 24 hours of diagnosis, to eradicate nasopharyngeal carriage and prevent secondary cases. Acceptable regimens include oral rifampin for 2 days, a single oral dose of ciprofloxacin, or a single intramuscular dose of ceftriaxone. Casual contacts, such as classmates without direct exposure to respiratory secretions, do not require prophylaxis. Vaccination is recommended for unvaccinated close contacts during outbreaks but does not replace antibiotic chemoprophylaxis for individual case contacts because it does not provide immediate protection.

• B. Amoxicillin is not an effective agent for meningococcal chemoprophylaxis and is not a recommended regimen.
• C. Close contacts require prophylaxis regardless of symptoms, because the goal is to prevent secondary disease, not just treat it.
• D. Vaccination alone does not provide immediate protection and does not replace chemoprophylaxis for close contacts.
• E. A prolonged intravenous antibiotic course is not indicated for asymptomatic prophylaxis and ceftriaxone for this indication is given as a single dose if used at all.

This patient has had two clinically distinct neurologic episodes separated in time (optic neuritis now, sensory symptoms 6 months ago) and anatomically separated in space (optic nerve and spinal cord), raising suspicion for multiple sclerosis. MRI of the brain and spinal cord with contrast is the best test because it can directly demonstrate dissemination in space (lesions in at least two of four typical CNS locations: periventricular, cortical/juxtacortical, infratentorial, spinal cord) and dissemination in time (presence of both enhancing and non-enhancing lesions, or new lesions on follow-up imaging), which are central to the current McDonald diagnostic criteria. CSF oligoclonal bands can support the diagnosis but are not sufficient alone and are used as a supplementary criterion when MRI findings are equivocal. Nerve conduction studies assess peripheral nerves and are not useful for a demyelinating CNS disease.

• B. Visual evoked potentials can show optic nerve conduction delay but cannot establish dissemination in space and time required for diagnosis.
• C. Oligoclonal bands alone support the diagnosis but cannot establish dissemination in space, which requires imaging.
• D. Nerve conduction studies evaluate peripheral nerve function and are irrelevant to a central demyelinating process.
• E. CT does not have adequate sensitivity to detect the white matter demyelinating lesions needed to diagnose MS.

This patient has an acute MS relapse, evidenced by new focal neurologic deficits corresponding to a new enhancing lesion on MRI, with infection excluded as a mimicking cause. High-dose intravenous corticosteroids (typically methylprednisolone 1 g daily for 3-5 days) are the first-line treatment for acute relapses because they reduce the duration and severity of the attack by decreasing inflammation at the site of demyelination, although they do not alter long-term disability progression. Plasma exchange is reserved for relapses that are severe and fail to respond adequately to corticosteroids. Disease-modifying therapies such as interferon beta-1a are used for long-term relapse prevention, not for treating an acute attack already in progress.

• A. Baclofen treats spasticity, a chronic symptom of MS, not the acute inflammatory relapse itself.
• C. IVIG is not a standard first-line therapy for acute MS relapses.
• D. Interferon beta-1a is a disease-modifying therapy for reducing future relapse frequency, not for treating a relapse already underway.
• E. Plasma exchange is reserved as second-line therapy for steroid-refractory relapses, not as initial treatment.

This patient has Guillain-Barré syndrome, confirmed by the classic presentation of ascending, symmetric, areflexic weakness following a diarrheal (likely Campylobacter jejuni) illness and albuminocytologic dissociation on CSF analysis (elevated protein with normal cell count). First-line treatment is IVIG or plasma exchange, which are equally effective and work by modulating the autoimmune attack on peripheral nerve myelin. IVIG is often preferred for ease of administration. Corticosteroids have not been shown to improve outcomes in GBS and are not recommended.

• A. Azathioprine is used for chronic immune-mediated neuropathies (e.g., CIDP), not acute GBS.
• C. Rituximab has no established role in acute GBS management.
• D. Corticosteroids, including oral prednisone, have not demonstrated benefit in GBS and are not part of standard therapy.
• E. IV methylprednisolone similarly lacks proven efficacy in GBS, unlike in some other inflammatory neuropathies.

This patient is in myasthenic crisis, precipitated by an aminoglycoside (which impairs neuromuscular transmission), with impending respiratory failure indicated by tachypnea, accessory muscle use, hypoxia, and a forced vital capacity under 15-20 mL/kg with reduced negative inspiratory force. The immediate priority is securing the airway with intubation and mechanical ventilation before pursuing disease-specific therapy. IVIG or plasmapheresis are then used to treat the underlying crisis, but airway protection takes precedence over these slower-acting therapies. Corticosteroids can transiently worsen weakness and are not first-line emergent therapy.

• A. IVIG is appropriate definitive therapy but takes days to work and does not address the immediate risk of respiratory arrest.
• B. High-dose corticosteroids can paradoxically worsen myasthenic weakness initially and are not the emergent priority.
• C. Plasmapheresis is effective for myasthenic crisis but is not the immediate step when the airway is not yet secured.
• E. Increasing pyridostigmine can worsen crisis by causing cholinergic excess and is not appropriate with impending respiratory failure.

The diagnosis of Parkinson disease is clinical, based on bradykinesia plus either resting tremor or rigidity, as in this patient who has a classic unilateral resting tremor that diminishes with action, cogwheel rigidity, bradykinesia with progressive decrement on repetitive movements, and asymmetric onset. Reduced facial expression (hypomimia), decreased arm swing, and shuffling gait further support the diagnosis. Early asymmetry and the absence of early postural instability or atypical features (early falls, vertical gaze palsy, autonomic failure) help distinguish idiopathic Parkinson disease from atypical parkinsonian syndromes.

• A. Normal pressure hydrocephalus presents with the triad of gait disturbance, urinary incontinence, and cognitive decline, without a classic resting tremor.
• C. Multiple system atrophy features early autonomic dysfunction (orthostatic hypotension, urinary incontinence) and cerebellar or more symmetric parkinsonian signs.
• D. Progressive supranuclear palsy typically presents with early falls, postural instability, and vertical gaze palsy, none of which are present here.
• E. Essential tremor is a bilateral action/postural tremor that worsens with voluntary movement, not a resting tremor that improves with action.

Long-term levodopa therapy commonly leads to motor complications, including peak-dose dyskinesias, which are choreiform or dystonic involuntary movements that occur when striatal dopamine levels are highest, typically about 1 hour after dosing. This results from pulsatile stimulation of dopamine receptors and denervation supersensitivity after years of levodopa therapy, and risk increases with cumulative dose and disease duration. Management includes reducing individual levodopa doses with more frequent dosing, adding amantadine, or adjusting adjunct therapy. This is distinct from tardive dyskinesia, which results from chronic dopamine receptor blockade by antipsychotics, not dopamine replacement.

• B. Essential tremor is a postural/action tremor, not choreiform movements tied temporally to dosing.
• C. Tardive dyskinesia results from chronic antipsychotic (dopamine receptor blocking) use, not levodopa, which this patient is not taking.
• D. Restless legs syndrome causes an urge to move the legs, typically worse at rest and in the evening, not dose-related involuntary movements.
• E. Serotonin syndrome presents with hyperthermia, clonus, and autonomic instability, typically from serotonergic drug combinations, not isolated levodopa use.

This patient has essential tremor, characterized by a symmetric, higher-frequency (usually 4-12 Hz) postural and kinetic tremor that worsens with sustained posture and action, improves with alcohol, is absent at rest, and often has a positive family history, in contrast to Parkinson disease's asymmetric low-frequency resting tremor with bradykinesia and rigidity. First-line pharmacotherapy for essential tremor is a non-selective beta-blocker such as propranolol, which reduces tremor amplitude through beta-adrenergic blockade; primidone is an alternative first-line agent. Dopaminergic and anticholinergic agents used in Parkinson disease have no proven benefit in essential tremor.

• A. Trihexyphenidyl, an anticholinergic, is used for Parkinsonian resting tremor, not the postural/kinetic tremor of essential tremor.
• B. Amantadine has mild antiparkinsonian activity but is not effective for essential tremor.
• C. Levodopa-carbidopa is used for Parkinson disease, which this patient does not have given the absence of bradykinesia, rigidity, or resting tremor.
• E. Pramipexole, a dopamine agonist, is used for Parkinson disease and has no established role in essential tremor treatment.

Huntington disease is caused by an expanded CAG trinucleotide repeat in the huntingtin (HTT) gene on chromosome 4, inherited in an autosomal dominant pattern with full penetrance. The repeat tends to expand further with paternal transmission, causing anticipation, in which successive generations develop symptoms at an earlier age and with greater severity. The clinical triad is chorea, psychiatric disturbance, and subcortical dementia, typically presenting in the 30s to 40s, as seen in this patient and his affected mother. Genetic testing for CAG repeat number confirms the diagnosis.

• A. Mitochondrial inheritance is strictly maternal and would not explain transmission from an affected mother to a son who then has his own disease, though it would not pass to his children from him; this patient's presentation and family history fit CAG repeat expansion instead.
• B. Mitochondrial point mutations cause disorders like MELAS or Leber hereditary optic neuropathy, not chorea with this family pattern.
• D. X-linked recessive dystrophin deletion causes Duchenne muscular dystrophy, affecting males with no father-to-son transmission.
• E. PMP22 duplication (not triplication) causes Charcot-Marie-Tooth disease type 1A, a demyelinating peripheral neuropathy.

This patient has cauda equina syndrome, indicated by bilateral leg symptoms, saddle anesthesia, urinary retention with elevated post-void residual, and reduced anal sphincter tone, most often caused by a large central disc herniation compressing multiple lumbosacral nerve roots. This is a surgical emergency requiring emergent MRI of the lumbosacral spine to confirm the diagnosis and localize the compressive lesion, followed by urgent surgical decompression, ideally within 24-48 hours, to prevent permanent bowel, bladder, and lower extremity dysfunction. Delaying imaging or treatment risks irreversible neurologic deficits. Bladder and bowel dysfunction are the findings most predictive of poor outcome if decompression is delayed.

• A. Conservative outpatient management is inappropriate given the acute neurologic emergency with bowel and bladder involvement.
• C. Delaying imaging risks permanent neurologic damage; cauda equina syndrome requires emergent, not outpatient, evaluation.
• D. Epidural steroid injection does not relieve mechanical compression and is not appropriate for this surgical emergency.
• E. Lumbar puncture does not evaluate for a structural compressive lesion and is not indicated here.

This patient's fever, severe focal back pain, elevated inflammatory markers, and new lower extremity weakness in the setting of intravenous drug use are classic for spinal epidural abscess, most commonly caused by Staphylococcus aureus from hematogenous spread. Urgent MRI with contrast is the most sensitive and specific imaging modality and can rapidly localize the abscess and assess for spinal cord compression, which is critical because neurologic deficits can progress to irreversible paralysis within hours. Blood cultures should be obtained but should not delay imaging, and empiric antibiotics plus neurosurgical consultation for possible decompression should follow promptly based on MRI findings. Plain radiographs lack the sensitivity to detect epidural abscess or early cord compression.

• A. Plain radiographs cannot visualize the epidural space and are insensitive for detecting an abscess or cord compression.
• C. Needle biopsy is not the initial step and should follow imaging to guide any further diagnostic sampling.
• D. Lumbar puncture risks introducing infection or causing herniation and is not the diagnostic test of choice when epidural abscess is suspected.
• E. Outpatient management is inappropriate for a patient with new neurologic deficits and signs of possible cord compression.

This patient has carpal tunnel syndrome, diagnosed clinically by nocturnal paresthesias in the median nerve distribution (thumb, index, middle fingers) along with positive Tinel sign (tapping reproduces tingling) and Phalen sign (wrist flexion reproduces symptoms). For mild to moderate symptoms without significant weakness or atrophy, conservative management with nighttime wrist splinting in a neutral position is the appropriate first-line treatment, as it reduces carpal tunnel pressure and often improves symptoms within weeks. Nerve conduction studies and electromyography are useful to confirm the diagnosis or assess severity when the diagnosis is unclear or symptoms persist despite conservative treatment, but they are not required before starting splinting. Surgery is reserved for patients with persistent symptoms despite conservative therapy or evidence of thenar muscle wasting.

• A. Nerve conduction studies can support the diagnosis but are not mandatory before starting conservative treatment in a classic clinical presentation.
• B. Surgery is reserved for patients who fail conservative management or who have signs of severe nerve damage such as thenar atrophy.
• C. Oral corticosteroids are not standard first-line therapy and carry systemic side effects not justified for mild symptoms.
• E. Corticosteroid injection can be considered if splinting fails but is not the initial step in an uncomplicated, mild presentation.

This patient has Bell palsy, an idiopathic lower motor neuron facial nerve palsy that characteristically involves the forehead, unlike a central (upper motor neuron) facial palsy from stroke, which spares forehead movement because of bilateral cortical innervation to the frontalis muscle. The involvement of her forehead along with normal strength and sensation elsewhere confirms a peripheral facial nerve lesion rather than stroke, so thrombolytics are not indicated. Oral corticosteroids (such as prednisone) started within 72 hours of symptom onset improve the rate and speed of recovery and are the standard treatment; antivirals alone are not recommended, though some guidelines support adding antivirals to steroids in severe cases. Most patients recover substantially within 3-6 months.

• A. Botulinum toxin is used for chronic synkinesis or facial spasm complications, not for acute Bell palsy.
• C. Alteplase is used for acute ischemic stroke, which this is not, given the forehead involvement indicating a peripheral rather than central lesion.
• D. Antiviral monotherapy without corticosteroids has not been shown to improve outcomes and is not standard treatment.
• E. Plasma exchange and high-dose IV steroids are used for conditions like Guillain-Barré syndrome or myasthenic crisis, not Bell palsy.

This patient's presentation of brief, severe, electric shock-like unilateral facial pain triggered by light touch or chewing, with a normal neurologic examination between episodes, is classic for trigeminal neuralgia, often caused by vascular compression of the trigeminal nerve root as seen on her MRI. Carbamazepine is the first-line pharmacologic treatment, acting by blocking sodium channels to reduce the hyperexcitable, ectopic firing of the trigeminal nerve, and most patients experience significant pain relief. Oxcarbazepine is a reasonable alternative with a similar mechanism and fewer drug interactions. Surgical options such as microvascular decompression are reserved for patients who fail or cannot tolerate medical therapy.

• A. NSAIDs are not effective for the neuropathic, paroxysmal pain characteristic of trigeminal neuralgia.
• B. Triptans are used for migraine and cluster headache, not trigeminal neuralgia.
• C. Baclofen can be used as an adjunct or second-line agent but is not the first-line treatment.
• E. Topiramate is not first-line for trigeminal neuralgia, though it may be used as an adjunct in refractory cases.

This patient has painful diabetic peripheral neuropathy, a common microvascular complication of long-standing diabetes presenting with symmetric, distal, burning dysesthesias that are worse at night. First-line pharmacotherapy includes the alpha-2-delta calcium channel ligands (pregabalin or gabapentin), serotonin-norepinephrine reuptake inhibitors (duloxetine), or tricyclic antidepressants, all of which have demonstrated efficacy in reducing neuropathic pain. Pregabalin works by binding to the alpha-2-delta subunit of voltage-gated calcium channels, reducing excitatory neurotransmitter release, and is FDA-approved specifically for diabetic peripheral neuropathy. Glycemic control should also be optimized, as better control slows the progression of neuropathy, but it does not address pain already present.

• A. NSAIDs are not effective for neuropathic pain, which has a different mechanism from nociceptive or inflammatory pain.
• B. Opioids are not recommended as first-line therapy for diabetic neuropathy given limited efficacy and risk of dependence.
• C. Phenytoin is not a standard treatment for diabetic peripheral neuropathy pain.
• E. Topical lidocaine can be used as adjunct therapy for localized pain but is not preferred as monotherapy for diffuse bilateral symptoms.

Before diagnosing Alzheimer disease, guidelines recommend a basic laboratory and sometimes imaging workup to exclude reversible or treatable causes of cognitive decline, including hypothyroidism, vitamin B12 deficiency, and electrolyte or metabolic disturbances. Structural neuroimaging (CT or MRI) is also obtained to exclude mass lesions, subdural hematoma, or normal pressure hydrocephalus. Alzheimer disease remains a clinical diagnosis of exclusion after these reversible causes have been ruled out. Only after this workup is completed should a clinical diagnosis of Alzheimer disease be made and treatment initiated.

• A. Neuropsychological testing is useful for staging but does not replace excluding metabolic or structural causes first.
• B. CSF amyloid/tau testing is a specialized, not first-line, test and is not needed before basic screening.
• C. Pharmacotherapy should not be started before excluding reversible causes of cognitive decline.
• D. EEG is not part of the routine dementia workup unless seizures are suspected.

Cholinesterase inhibitors such as donepezil, rivastigmine, and galantamine are first-line therapy for mild-to-moderate Alzheimer disease. They act by inhibiting acetylcholinesterase, increasing synaptic acetylcholine levels to partially compensate for the cholinergic neuronal loss characteristic of the disease. They provide modest symptomatic improvement in cognition and function but do not reverse or halt the underlying neurodegeneration. Memantine, an NMDA receptor antagonist, is typically added in moderate-to-severe disease rather than used as monotherapy for mild-to-moderate disease.

• A. Sertraline may help comorbid depression but does not address the underlying cognitive decline of Alzheimer disease.
• B. Memantine is indicated for moderate-to-severe Alzheimer disease, not as first-line therapy for mild-to-moderate disease.
• C. Levodopa-carbidopa treats parkinsonian motor symptoms and has no role in Alzheimer disease.
• E. Antipsychotics are not indicated for cognitive decline and carry increased mortality risk in dementia patients.

This patient has the classic triad of normal pressure hydrocephalus: gait disturbance (typically the earliest and most prominent feature), urinary incontinence, and cognitive decline, with ventriculomegaly disproportionate to cortical atrophy on imaging. Gait improvement after a large-volume lumbar puncture ('tap test') supports the diagnosis and predicts response to treatment. Ventriculoperitoneal shunting is the definitive treatment and can significantly improve gait and other symptoms, especially when gait disturbance predominates and is treated early. Medical therapy has no established role in management of this condition.

• A. Levodopa-carbidopa is used for Parkinson disease, which presents with resting tremor and rigidity rather than this triad with ventriculomegaly.
• C. CSF cytology is used to evaluate for malignancy or infection, not to treat normal pressure hydrocephalus.
• D. Donepezil is used for Alzheimer disease, not normal pressure hydrocephalus.
• E. Acetazolamide reduces CSF production in conditions like idiopathic intracranial hypertension, not normal pressure hydrocephalus.

Vascular dementia is characterized by a stepwise, fluctuating decline in cognition that correlates temporally with cerebrovascular events, in contrast to the slow, steadily progressive decline typical of Alzheimer disease. Risk factors include hypertension, diabetes, and prior stroke, and focal neurologic signs such as hemiparesis or gait abnormality are common because of underlying infarcts. Imaging showing multiple lacunar infarcts or significant white matter disease supports the diagnosis. Management focuses on controlling vascular risk factors to prevent further decline, as there is no specific disease-modifying therapy.

• A. Frontotemporal dementia typically presents with early personality and behavioral changes rather than a stepwise decline after strokes.
• C. Alzheimer disease causes a gradual, continuous decline without the stepwise pattern or stroke-related imaging findings seen here.
• D. Normal pressure hydrocephalus presents with gait disturbance, incontinence, and dementia with ventriculomegaly, not stepwise decline after strokes.
• E. Lewy body dementia presents with visual hallucinations, parkinsonism, and fluctuating cognition, not a stepwise decline tied to strokes.

Lewy body dementia is characterized by fluctuating cognition, recurrent visual hallucinations, and spontaneous parkinsonism, often with cognitive decline preceding or occurring within one year of motor symptom onset. These patients have marked sensitivity to antipsychotics, which can precipitate severe, sometimes irreversible, extrapyramidal reactions and worsening confusion due to dopamine receptor blockade in a brain already depleted of dopaminergic neurons. This severe antipsychotic sensitivity is a distinguishing clinical clue supporting the diagnosis. Management favors cautious use of cholinesterase inhibitors for cognitive symptoms and avoidance of typical and most atypical antipsychotics.

• A. Creutzfeldt-Jakob disease progresses much more rapidly over weeks to months and typically includes myoclonus, not this presentation.
• C. Alzheimer disease does not typically cause early visual hallucinations, parkinsonism, or marked antipsychotic sensitivity.
• D. Delirium from infection would not explain the chronic progressive course over a year or the parkinsonian features.
• E. In Parkinson disease dementia, motor symptoms precede cognitive decline by more than a year, unlike the near-simultaneous onset here.

This patient's acute onset of confusion with fluctuating attention and inattention, superimposed on a stable baseline, is characteristic of delirium rather than progression of dementia, which develops gradually over months to years. Delirium is common postoperatively and is frequently multifactorial, with precipitants including infection, hypoxia, electrolyte disturbances, pain, and medication effects (including opioids and anticholinergics). The most appropriate next step is a focused evaluation for reversible precipitating causes, such as checking for infection (elevated temperature and heart rate), reviewing oxygenation (saturation 94%), and reviewing the medication list. Identifying and treating the underlying cause is the cornerstone of delirium management.

• A. This presentation is not expected and reflects delirium, which requires active evaluation, not reassurance.
• C. Donepezil treats chronic dementia and has no role in the acute management of delirium.
• D. Antipsychotics are reserved for severe agitation posing safety risks after nonpharmacologic measures and should not be started as a first step without addressing the underlying cause.
• E. MRI may be considered if an initial workup is unrevealing or a focal neurologic deficit is found, but it is not the first step.

This patient has signs of elevated intracranial pressure and impending herniation, including a dilated, poorly reactive pupil (suggesting uncal herniation) and the Cushing reflex (hypertension, bradycardia, irregular respiration). Immediate temporizing measures while arranging definitive surgical evacuation include elevating the head of the bed to 30 degrees to promote venous drainage and administering hyperosmolar therapy, such as mannitol or hypertonic saline, to osmotically draw free water out of brain tissue and reduce intracranial pressure. These measures can be lifesaving bridges to definitive neurosurgical decompression. Hyperventilation may also be used transiently in a crisis, but routine prolonged hyperventilation is avoided.

• A. Corticosteroids are not indicated for cytotoxic edema from traumatic brain injury and have not shown benefit in this setting.
• B. Trendelenburg position increases intracranial pressure by impairing venous drainage and is contraindicated.
• D. Lumbar puncture is contraindicated in suspected elevated intracranial pressure with mass effect because of the risk of precipitating herniation.
• E. Large-volume isotonic fluid boluses do not lower intracranial pressure and are not the priority intervention here.

The apnea test is a required confirmatory component of brain death determination after prerequisites (normothermia, normotension, no sedating drugs, no severe metabolic derangements) and a full clinical examination showing absent brainstem reflexes and coma with no cranial nerve function are met. The test is positive for brain death when the arterial PCO2 rises to at least 60 mm Hg (or 20 mm Hg above baseline) without any observed respiratory effort, demonstrating failure of the medullary respiratory center to respond to a maximal hypercapnic stimulus. PO2 and pH changes are not the defining criterion; CO2 is the specific stimulus for the respiratory centers. Any respiratory movement, even a single gasp, invalidates the test as showing brainstem function and must be reported as a negative apnea test.

• B. A PCO2 that stays low indicates inadequate hypercapnic stimulus and an indeterminate, not confirmatory, test.
• C. A falling pH reflects respiratory acidosis from CO2 retention but is not itself the defining confirmatory threshold.
• D. Hypoxemia is not the diagnostic threshold for the apnea test; PCO2 rise is the specific criterion used.
• E. Any observed respiratory effort, including a single gasp, indicates a negative apnea test regardless of PCO2 level.