USMLE Step 2 CK emergency medicine & toxicology questions

The osmolar gap is the difference between measured and calculated serum osmolality; a gap greater than 10 mOsm/kg suggests the presence of an unmeasured osmotically active substance such as a toxic alcohol (ethylene glycol in this case, from antifreeze). Here the gap is 320 minus 288, or 32 mOsm/kg, strongly supporting toxic alcohol ingestion while a specific ethylene glycol level is pending. Urine immunoassays do not detect toxic alcohols. Serum lactate dehydrogenase, beta-hydroxybutyrate, and creatine kinase are not used to screen for toxic alcohol ingestion.

• A. LDH is not used to screen for toxic alcohol ingestion.
• B. Creatine kinase assesses muscle injury, not toxic alcohol exposure.
• D. Beta-hydroxybutyrate assesses ketosis, not toxic alcohol exposure.
• E. Standard urine drug immunoassays do not detect ethylene glycol or methanol.

Iron tablets are radiopaque and not adsorbed by activated charcoal, making whole bowel irrigation with polyethylene glycol electrolyte solution the preferred decontamination method for large iron ingestions or ingestions of sustained-release or enteric-coated agents visible on imaging. It works by mechanically flushing tablets through the gastrointestinal tract before significant absorption occurs. Deferoxamine chelation is reserved for patients with systemic toxicity (shock, severe acidosis, markedly elevated serum iron levels), which this child does not yet have. Gastric lavage and endoscopy are not effective for multiple small tablets scattered through the bowel.

• A. Endoscopy is impractical for multiple tablets distributed throughout the small bowel.
• C. Deferoxamine is reserved for evidence of systemic iron toxicity, not used empirically here.
• D. Activated charcoal does not bind iron and is ineffective.
• E. Gastric lavage is ineffective for tablets that have already passed into the small bowel.

The Rumack-Matthew nomogram plots serum acetaminophen concentration against time since a single acute ingestion, using a treatment line starting at 150 mcg/mL at 4 hours; levels above this line predict a significant risk of hepatotoxicity and indicate treatment with N-acetylcysteine should begin. The nomogram is only valid for levels drawn between 4 and 24 hours after a known acute single ingestion, and 6 hours falls within this valid window. Levels drawn before 4 hours are unreliable because absorption may be incomplete, but that is not the issue here. Delaying treatment for a repeat level would unnecessarily postpone therapy in a patient already above the treatment threshold.

• A. The level is above the treatment line, indicating significant hepatotoxicity risk, not a safe ingestion.
• C. The level was drawn at 6 hours, which is within the valid interpretive window of 4-24 hours.
• D. Treatment should not be delayed when the level already clearly falls above the treatment line.
• E. The ingestion time is known in this case (stated as 6 hours prior), so the nomogram can be applied.

Salicylate toxicity classically produces a mixed acid-base disturbance: early respiratory alkalosis from direct stimulation of the medullary respiratory center causing tachypnea and hyperventilation, combined with an underlying high anion gap metabolic acidosis from accumulation of organic acids and uncoupling of oxidative phosphorylation. The nearly normal pH with low PaCO2 and low bicarbonate reflects this mixed picture, and tinnitus is a hallmark early symptom of salicylate toxicity. DKA typically presents with hyperglycemia, which is absent here. Methanol toxicity causes a pure high anion gap metabolic acidosis with visual disturbances rather than tinnitus, and opioid overdose causes respiratory depression with hypoventilation, which is inconsistent with this patient's deep, rapid breathing.

• A. Glucose is normal, excluding diabetic ketoacidosis.
• B. Opioid overdose causes respiratory depression with hypoventilation, not tachypnea with deep breathing.
• C. Carbon monoxide poisoning causes headache and altered mental status, not tinnitus with this acid-base pattern.
• D. Methanol causes visual disturbances rather than tinnitus and does not produce this classic mixed respiratory alkalosis/metabolic acidosis pattern.

Salicylate is a weak acid, and alkalinizing the urine with intravenous sodium bicarbonate increases the proportion of salicylate that exists in its ionized form within the renal tubules, which cannot be reabsorbed back into the bloodstream (ion trapping). This increases urinary excretion of salicylate and its elimination from the body. The target urine pH is typically 7.5-8.0, and serum pH and potassium must be monitored since hypokalemia impairs the ability to alkalinize urine. Sodium bicarbonate does not directly bind salicylate, induce hepatic or renal enzymes, or cause redistribution into fat tissue.

• A. Bicarbonate does not form a direct chemical complex with salicylate in the blood.
• C. Salicylate is lipophilic in its non-ionized form, so alkalinization reduces rather than promotes tissue redistribution.
• D. Alkalinization does not increase hepatic metabolism of salicylate.
• E. Sodium bicarbonate does not induce cytochrome P450 enzymes to degrade salicylate.

Methanol is metabolized by alcohol dehydrogenase to formaldehyde and then by aldehyde dehydrogenase to formic acid, which is directly toxic to the retina and optic nerve, producing the classic "snowstorm" visual disturbance and hyperemic optic disc. This toxic metabolite also causes a severe high-anion-gap metabolic acidosis with an elevated osmolar gap, as seen in this patient. The combination of visual symptoms, optic disc findings, and acidosis with a wide osmolar gap after a suspected toxic ingestion is characteristic of methanol poisoning. Early recognition is critical because formic acid accumulation can cause permanent blindness and death if treatment is delayed.

• A. Ethylene glycol causes flank pain and renal injury with calcium oxalate crystals, not visual disturbance.
• C. DKA presents with hyperglycemia and ketosis, not visual snowstorm symptoms or an elevated osmolar gap from ingestion.
• D. Ethanol intoxication does not cause an anion gap metabolic acidosis or retinal toxicity.
• E. Isopropyl alcohol causes ketosis without a significant anion gap elevation and does not cause visual loss.

Ethylene glycol is metabolized by alcohol dehydrogenase to glycolaldehyde and ultimately to glycolic and oxalic acid, producing a severe high-anion-gap metabolic acidosis. Oxalic acid combines with calcium to form calcium oxalate crystals, which precipitate in the renal tubules as the characteristic envelope-shaped (or needle-shaped) crystals seen on urinalysis, causing acute kidney injury and hypocalcemia. This triad of metabolic acidosis, acute kidney injury, and calcium oxalate crystalluria after a known antifreeze ingestion is diagnostic of ethylene glycol toxicity. Fomepizole should be started promptly to block further toxic metabolite formation.

• A. Methanol causes visual toxicity and formic acid accumulation, not calcium oxalate crystalluria or renal failure.
• C. Rhabdomyolysis causes reddish-brown urine with myoglobin, not envelope-shaped crystals, and is not linked to antifreeze ingestion.
• D. Ethanol-induced ketoacidosis causes ketosis without crystalluria or significant renal failure.
• E. Salicylate poisoning causes a mixed acid-base disorder with tinnitus, not crystalluria or hypocalcemia.

Fomepizole is a competitive inhibitor of alcohol dehydrogenase, the enzyme responsible for converting toxic alcohols such as ethylene glycol and methanol into their harmful metabolites (glycolic/oxalic acid and formic acid, respectively). By blocking this enzyme, fomepizole prevents further generation of toxic metabolites while allowing the unmetabolized parent alcohol to be slowly cleared by the kidneys or removed by dialysis. It has a much higher affinity for alcohol dehydrogenase than ethanol does, making it the preferred antidote over ethanol infusion. Fomepizole does not reverse already-formed toxic metabolites, so early administration is essential.

• B. Fomepizole does not act on renal transporters or excretion pathways.
• C. Fomepizole does not bind or chelate toxic metabolites once they are formed.
• D. Fomepizole inhibits, rather than induces, the relevant metabolic enzyme pathway.
• E. Fomepizole has no direct buffering or neutralizing effect on formic acid.

Hemodialysis is indicated in toxic alcohol poisoning for severe metabolic acidosis (pH less than 7.25-7.30), visual disturbances or other end-organ toxicity, a markedly elevated toxic alcohol level (generally methanol or ethylene glycol greater than 50 mg/dL), or renal impairment, as seen in this patient. Dialysis both removes the parent toxic alcohol and corrects the severe acidosis more rapidly than metabolic clearance alone, while fomepizole is continued to prevent ongoing toxic metabolite formation. This patient meets multiple criteria: severe acidosis, visual symptoms, a methanol level well above 50 mg/dL, and acute kidney injury. Delaying dialysis risks permanent visual loss and worsening acidosis.

• A. Increasing fomepizole dose alone will not remove already-accumulated toxic alcohol or correct severe acidosis quickly enough.
• B. Observation is inappropriate given severe acidosis, visual symptoms, and a markedly elevated methanol level requiring urgent removal.
• C. Activated charcoal does not effectively adsorb small alcohol molecules and is not indicated.
• D. Ethanol infusion is an alternative to fomepizole for enzyme inhibition but does not substitute for dialysis in severe toxicity and is not preferred when fomepizole is available.

Tricyclic antidepressants block fast cardiac sodium channels, slowing phase 0 depolarization and widening the QRS complex, which predisposes to ventricular arrhythmias. Sodium bicarbonate works through two complementary mechanisms: the sodium load increases the extracellular-to-intracellular sodium gradient, which helps overcome the channel blockade and improve conduction, and the resulting alkalosis reduces the drug's affinity for the sodium channel. This dual effect narrows the QRS complex and reduces arrhythmia risk, and is the first-line treatment for TCA-induced cardiotoxicity with QRS widening greater than 100 msec or ventricular arrhythmia. Hypertonic saline can provide a similar sodium-gradient benefit but does not address the alkalosis component.

• B. Sodium bicarbonate does not chemically bind or inactivate the tricyclic antidepressant molecule.
• C. The cardiac benefit of bicarbonate is due to sodium channel effects, not increased renal clearance of the drug.
• D. Sodium bicarbonate does not act on hepatic enzymes to increase drug metabolism.
• E. TCA toxicity is not primarily related to potassium disturbance, and bicarbonate's benefit is not through potassium correction.

Digoxin toxicity classically presents with gastrointestinal symptoms (nausea, vomiting), visual disturbances (yellow-green halos), and cardiac conduction abnormalities, including bradyarrhythmias such as regularized atrial fibrillation with a slow ventricular response, which suggests a junctional escape rhythm from enhanced vagal tone and suppressed AV conduction. The scooped ("Salvador Dali mustache") ST depression is a chronic digoxin effect, but acute toxicity is precipitated here by renal impairment from a new diuretic reducing digoxin clearance and hypokalemia, which potentiates digoxin's binding to the Na+/K+-ATPase pump. The combination of visual symptoms, GI upset, bradyarrhythmia, and risk factors (renal injury, hypokalemia) in a patient on digoxin is diagnostic of digoxin toxicity.

• A. This patient is not described as taking a beta-blocker, and beta-blocker toxicity does not cause visual halos.
• B. ACS typically presents with chest pain and ischemic ECG changes rather than visual halos and a regularized junctional bradyarrhythmia.
• D. Hypokalemia alone does not explain the visual symptoms or the characteristic regularized bradyarrhythmia seen with digoxin.
• E. Decompensated heart failure would more typically present with dyspnea and volume overload rather than visual halos and a regularized bradyarrhythmia.

Glucagon is the preferred agent for beta-blocker overdose refractory to fluids and atropine because it increases intracellular cyclic AMP through a receptor independent of the beta-adrenergic pathway, directly activating adenylate cyclase. This bypasses the blocked beta-receptor and restores inotropy and chronotropy, improving heart rate and blood pressure, and can also help correct the hypoglycemia often seen in beta-blocker toxicity due to impaired glycogenolysis. Common side effects include nausea and vomiting, so airway protection should be considered before administration in obtunded patients. High-dose insulin-euglycemia therapy and calcium are additional options if glucagon fails, but glucagon is the standard first pharmacologic step after failed supportive measures.

• A. Isoproterenol acts on beta-receptors, which are blocked in this overdose, making it less effective and also risks exacerbating hypotension.
• B. Digoxin immune Fab is specific for digoxin toxicity, not beta-blocker overdose.
• C. Naloxone reverses opioid toxicity, not beta-blocker-induced bradycardia and hypotension.
• E. Flumazenil reverses benzodiazepine toxicity, not beta-blocker toxicity, and there is no indication of benzodiazepine co-ingestion here.

Serotonin syndrome results from excess serotonergic activity, commonly from combining an SSRI with another serotonergic agent such as tramadol. It is characterized by the rapid onset (within 24 hours) of altered mental status, autonomic instability, and neuromuscular findings including hyperreflexia, tremor, and inducible or spontaneous clonus, especially prominent in the lower extremities. Neuroleptic malignant syndrome, in contrast, develops over days and features lead-pipe rigidity with decreased or normal reflexes rather than clonus and hyperreflexia. The combination of a recent serotonergic drug addition with clonus strongly favors serotonin syndrome over NMS.

• A. Malignant hyperthermia occurs intraoperatively following exposure to volatile anesthetics or succinylcholine, not from oral serotonergic medications.
• C. Alcohol withdrawal causes tremor and autonomic hyperactivity but does not produce clonus or occur in this medication-exposure context.
• D. Anticholinergic toxicity causes dry skin and absent bowel sounds rather than diaphoresis and hyperreflexia with clonus.
• E. NMS typically evolves over days to weeks after starting or increasing a dopamine antagonist and presents with lead-pipe rigidity and decreased reflexes, not clonus.

Hemodialysis is indicated in severe lithium toxicity with a level above 4.0 mEq/L, or above 2.5 mEq/L when significant renal impairment, neurologic symptoms, or hemodynamic instability are present, as in this patient with confusion, acute kidney injury, and a rising lithium level in the setting of NSAID-induced decreased renal clearance. Lithium is a small molecule with low protein binding and a small volume of distribution, making it readily dialyzable. Isotonic saline helps restore volume and renal perfusion but is insufficient alone when neurologic and renal findings indicate severe toxicity. Activated charcoal does not bind lithium because it is not an organic compound.

• A. Activated charcoal does not adsorb lithium because lithium is a small, fully ionized inorganic cation.
• B. Whole bowel irrigation is used for sustained-release preparations or substances not adsorbed by charcoal, but it does not address the current severe systemic toxicity requiring extracorporeal removal.
• C. Saline supports renal lithium clearance but is inadequate as sole therapy given this patient's neurologic symptoms and significant renal impairment.
• D. Sodium polystyrene sulfonate is used for hyperkalemia and has no established role in lithium toxicity.

Naloxone is a competitive antagonist at mu-opioid receptors, displacing opioid agonists and rapidly reversing respiratory depression, miosis, and sedation. It has a short half-life of 30-90 minutes, which can be shorter than that of the ingested opioid, so repeat dosing or an infusion may be required for longer-acting agents such as methadone. It can be administered intravenously, intramuscularly, or intranasally, allowing rapid use in the field. This mechanism explains why naloxone has no effect on respiratory depression from sedative-hypnotics or other non-opioid causes.

• B. Acetylcholinesterase inhibition describes drugs such as physostigmine or neostigmine, unrelated to opioid receptor antagonism.
• C. Alpha-2 agonism describes drugs such as clonidine or dexmedetomidine, which are not used to reverse opioid overdose.
• D. NMDA receptor antagonism describes the mechanism of drugs like ketamine, unrelated to opioid reversal.
• E. This describes flumazenil, which reverses benzodiazepine effects, not opioid toxicity.

Flumazenil is generally contraindicated in patients with chronic benzodiazepine use and in suspected co-ingestion of proconvulsant agents such as tricyclic antidepressants, because abrupt reversal of GABA-A mediated inhibition can precipitate seizures that may be refractory to benzodiazepine treatment once the receptor is blocked. This patient's widened QRS and history of chronic alprazolam use with suspected tricyclic co-ingestion place her at high risk for this complication. Management should instead focus on airway protection, supportive care, and treatment of tricyclic toxicity with sodium bicarbonate if cardiotoxicity develops. Flumazenil is better reserved for isolated, acute benzodiazepine overdose in a benzodiazepine-naive patient.

• A. Flumazenil is not considered first-line or definitive therapy; supportive airway management is prioritized, especially given the seizure risk here.
• B. Flumazenil does not cause a hypertensive crisis with tricyclics; the concern is seizure precipitation, not hypertension.
• D. Flumazenil and amitriptyline are not metabolized through a clinically significant shared pathway causing this described accumulation effect.
• E. Flumazenil has no effect on tricyclic cardiotoxicity and does not reverse sodium channel blockade.

Beta-blockers such as metoprolol should be avoided in cocaine-associated chest pain because blocking beta-2-mediated vasodilation leaves alpha-adrenergic vasoconstriction unopposed, which can worsen coronary vasospasm and hypertension. Benzodiazepines are first-line to reduce central sympathetic outflow, and nitroglycerin and calcium channel blockers such as diltiazem are preferred to treat vasospasm and hypertension. Phentolamine, an alpha-blocker, can be used for refractory hypertension because it directly opposes the alpha-adrenergic vasoconstriction driving the toxicity. Reperfusion therapy follows standard STEMI protocols once beta-blockers have been excluded from the regimen.

• A. Diltiazem is an acceptable alternative to treat hypertension and tachycardia without the unopposed alpha-adrenergic risk of beta-blockers.
• B. Morphine is appropriate for analgesia in ongoing ischemic chest pain when nitroglycerin is insufficient.
• C. Nitroglycerin is appropriate and recommended to relieve cocaine-induced coronary vasospasm.
• E. Phentolamine, an alpha-adrenergic antagonist, is appropriate for refractory hypertension in this setting.

This patient has a cholinergic toxidrome from organophosphate (pesticide) poisoning, producing SLUDGE symptoms (salivation, lacrimation, urination, defecation, GI distress, emesis) plus miosis, bronchorrhea, bradycardia, and muscle fasciculations from excess acetylcholine at muscarinic and nicotinic receptors. Atropine is a competitive muscarinic antagonist that reverses the life-threatening muscarinic effects, especially bronchorrhea and bradycardia, and should be titrated to drying of secretions. Pralidoxime is added to reactivate acetylcholinesterase and treat nicotinic effects such as fasciculations and weakness, but atropine takes priority for immediate stabilization.

• A. Neostigmine inhibits acetylcholinesterase and would worsen this patient's symptoms.
• B. Flumazenil reverses benzodiazepine toxicity, not cholinergic toxicity.
• D. Naloxone reverses opioid toxicity, which presents with miosis and respiratory depression but not salivation or fasciculations.
• E. Physostigmine inhibits acetylcholinesterase and would worsen cholinergic excess.

Carbon monoxide poisoning should be suspected with exposure to combustion in an enclosed space, headache, nausea, and altered mental status. Standard pulse oximetry cannot distinguish carboxyhemoglobin from oxyhemoglobin because the two have similar light absorption, so oxygen saturation reads falsely normal despite significant hypoxia at the tissue level. Co-oximetry directly measures carboxyhemoglobin fraction using multiple wavelengths and is the test that confirms the diagnosis and quantifies severity. A normal pulse oximetry reading in a patient with suspected CO poisoning should never be reassuring.

• A. Lactate may be elevated from tissue hypoxia but is nonspecific and does not confirm CO poisoning.
• B. Pulse oximetry cannot differentiate carboxyhemoglobin from oxyhemoglobin and gives a falsely normal reading.
• C. Blood gas pH and bicarbonate may show metabolic acidosis but do not confirm the diagnosis.
• E. Chest radiograph is not useful for diagnosing carbon monoxide poisoning.

Hyperbaric oxygen therapy is indicated in carbon monoxide poisoning when there is loss of consciousness, carboxyhemoglobin level greater than 25%, severe metabolic acidosis, cardiac ischemia or dysrhythmia, or pregnancy with a carboxyhemoglobin level greater than 20%. This patient had a syncopal episode and a carboxyhemoglobin level of 28%, both of which meet criteria for hyperbaric therapy, which accelerates carbon monoxide elimination and reduces delayed neurologic sequelae more effectively than normobaric oxygen alone. Normobaric oxygen should be given while arranging transfer but is not sufficient definitive therapy in this case.

• A. Exchange transfusion is not used for carbon monoxide poisoning.
• B. Normobaric oxygen alone is insufficient given her syncope and carboxyhemoglobin level above 25%, which meet criteria for hyperbaric therapy.
• C. Sodium bicarbonate is not indicated for treating carbon monoxide toxicity itself.
• E. Hydroxocobalamin treats cyanide toxicity, not carbon monoxide poisoning, and there is no indication of cyanide exposure here.

This patient has findings consistent with cyanide toxicity from smoke inhalation, including severe lactic acidosis out of proportion to his oxygenation status and hemodynamic instability, in the setting of a structural fire where burning synthetic materials release hydrogen cyanide. Hydroxocobalamin binds cyanide to form cyanocobalamin, which is renally excreted, and is preferred in smoke inhalation victims because it does not induce methemoglobinemia or hypotension the way nitrites do. Sodium nitrite is avoided in smoke inhalation because concurrent carbon monoxide poisoning and methemoglobinemia would further impair oxygen-carrying capacity.

• A. Sodium nitrite induces methemoglobinemia and is relatively contraindicated in smoke inhalation victims who may also have carbon monoxide poisoning.
• B. Deferoxamine chelates iron, not cyanide.
• C. Methylene blue treats methemoglobinemia, not cyanide toxicity.
• D. Pralidoxime reactivates acetylcholinesterase in organophosphate poisoning and has no role in cyanide toxicity.

Methemoglobinemia occurs when iron in hemoglobin is oxidized from the ferrous (Fe2+) to the ferric (Fe3+) state, impairing oxygen binding and causing cyanosis that does not improve with supplemental oxygen, as seen here after exposure to an oxidizing agent such as topical benzocaine. The classic clue is a normal PaO2 on arterial blood gas (reflecting dissolved oxygen) with a low pulse oximetry reading and a saturation gap, along with chocolate-brown colored blood. Methylene blue is the treatment, acting as a cofactor to reduce methemoglobin back to functional hemoglobin via NADPH-methemoglobin reductase.

• B. Pulmonary embolism would not explain the chocolate-brown blood or the specific oxidizing drug exposure.
• C. Carbon monoxide poisoning causes falsely normal, not low, pulse oximetry readings and is not linked to benzocaine exposure.
• D. Cyanide toxicity causes tissue hypoxia with normal-appearing bright red blood, not chocolate-brown blood.
• E. Sulfhemoglobinemia also causes oxygen-refractory cyanosis but is rare and not typically linked to benzocaine; methemoglobinemia is far more classically associated with topical anesthetics.

This child is in the second stage (latent/relative stability masking ongoing toxicity) progressing to the third stage (shock and metabolic acidosis) of iron overdose, with a markedly elevated serum iron level and systemic toxicity including hypotension and severe metabolic acidosis. Intravenous deferoxamine is the chelating agent of choice, binding free iron to form ferrioxamine, which is excreted in urine and classically turns it a vin rosé color. Activated charcoal does not adsorb iron and has no role in this overdose.

• B. Calcium disodium EDTA is used for lead poisoning, not iron overdose.
• C. Deferasirox is an oral iron chelator used for chronic iron overload, not for acute severe iron toxicity with shock.
• D. Dimercaprol is used for arsenic, mercury, and lead poisoning, not iron overdose.
• E. Activated charcoal does not bind iron and is not useful in iron overdose.

Heat stroke is defined by a core temperature generally above 40°C (104°F) accompanied by central nervous system dysfunction such as confusion, delirium, seizures, or coma, reflecting failure of thermoregulation. Heat exhaustion, in contrast, involves similar environmental exposure and elevated temperature but preserved mental status without significant neurologic impairment. This patient's altered mental status with markedly elevated temperature after exertional heat exposure confirms heat stroke, which requires immediate aggressive cooling, most effectively by cold water immersion, to reduce the risk of multiorgan damage.

• A. Neuroleptic malignant syndrome is associated with antipsychotic drug exposure and muscular rigidity, not exertional heat exposure during a marathon.
• B. Serotonin syndrome is associated with serotonergic drug exposure and features clonus and hyperreflexia, not exertional heat exposure.
• C. Thyroid storm presents with hyperthermia and tachycardia but is triggered by thyroid hormone excess or stressors in a hyperthyroid patient, not primarily by exertional heat exposure.
• D. Heat exhaustion does not cause significant central nervous system dysfunction such as confusion; mental status remains intact.

The treatment of choice for frostbite is rapid rewarming by immersion in a water bath maintained at 98.6–102.2°F (37–39°C) for 15–30 minutes until the tissue becomes pliable and red or purple. Rubbing or massaging frostbitten tissue is avoided because it causes additional mechanical trauma to already fragile, ice-crystal-damaged cells. Dry heat sources such as heating pads risk uneven heating and burns because the numb tissue cannot sense excessive temperature. Passive rewarming at ambient temperature is too slow and associated with worse tissue outcomes compared with controlled rapid rewarming.

• A. Slow ambient rewarming is associated with greater tissue loss than controlled rapid rewarming in water.
• B. Dry heat sources risk uneven or excessive heating and burns in numb, insensate tissue.
• C. Rubbing causes mechanical shearing of ice crystals through cell membranes, worsening tissue injury.
• E. Snow is below freezing and would not provide the warming required and risks further cold injury.

Even asymptomatic drowning victims can develop delayed noncardiogenic pulmonary edema (a form of acute respiratory distress syndrome) due to surfactant washout and alveolar-capillary membrane injury from aspirated water, with symptoms sometimes appearing up to 4–8 hours later. This is why guidelines recommend a minimum observation period of 4–6 hours with serial examination and pulse oximetry before considering discharge, even if the patient appears asymptomatic initially. Chest radiography and clinical reassessment help detect evolving pulmonary edema. Bacterial pneumonia can occur after drowning but typically develops over days, not hours, and is not the primary concern justifying short-term observation.

• B. Hemothorax requires traumatic injury to thoracic vessels, which is not suggested by this scenario.
• C. Pulmonary embolism is not a recognized complication of simple freshwater or pool submersion injury.
• D. Bacterial pneumonia, when it occurs after drowning, typically develops over days rather than within hours.
• E. Pneumothorax is not a typical complication of submersion injury in the absence of barotrauma or trauma.

Acetazolamide is the preferred agent for prevention of acute mountain sickness in travelers undertaking rapid ascent, typically started 1–2 days before ascent and continued for 2–3 days at altitude. It works by causing a bicarbonate diuresis that induces a mild metabolic acidosis, which stimulates ventilation and accelerates acclimatization. Dexamethasone is an alternative for prevention or treatment but is generally reserved for those who cannot tolerate acetazolamide or for treatment of severe AMS and high-altitude cerebral edema. Nifedipine is used for prevention and treatment of high-altitude pulmonary edema, not AMS, and ibuprofen can treat headache symptoms but does not prevent the underlying acclimatization failure.

• B. Ibuprofen may reduce headache symptoms but does not address the underlying mechanism of altitude maladaptation.
• C. Dexamethasone is an alternative for those intolerant of acetazolamide, not the first-line prophylactic choice.
• D. Sildenafil has a role in high-altitude pulmonary edema, not routine AMS prevention.
• E. Nifedipine prevents and treats high-altitude pulmonary edema, not acute mountain sickness.

Lightning delivers a massive, instantaneous direct current (DC)-like discharge that depolarizes the entire myocardium simultaneously, producing asystole; because the heart has inherent automaticity, cardiac activity often resumes spontaneously. However, the brainstem respiratory center can remain suppressed longer than the heart, causing a secondary respiratory arrest that leads to hypoxia and a true, prolonged cardiac arrest if not supported with ventilation. This distinguishes lightning injury from household alternating current (AC) electrical injury, which classically causes ventricular fibrillation rather than asystole. Prompt bag-mask ventilation or intubation is critical to prevent secondary hypoxic cardiac arrest.

• A. Ventricular fibrillation is the classic mechanism in AC (household current) exposure, not lightning, which behaves as a massive DC-like discharge.
• B. Tension pneumothorax is not a typical mechanism of lightning-induced arrest and there is no clinical evidence for it here.
• C. There is no blast mechanism in lightning strikes; the primary injury is electrical, not barotraumatic to lung parenchyma.
• E. Coronary dissection is not a recognized mechanism of lightning-induced cardiopulmonary arrest.

This patient has a pit viper envenomation with progressive local swelling, coagulopathy (thrombocytopenia and hypofibrinogenemia), and systemic signs, meeting criteria for antivenom administration. Crotalidae polyvalent immune Fab (CroFab or the newer Anavip) is indicated for progressive local tissue injury, hematologic abnormalities, or systemic toxicity following pit viper envenomation, and works by binding and neutralizing circulating venom components. Tourniquets, incision and suction, and ice application are outdated prehospital measures that can worsen local tissue injury and are not recommended. Fasciotomy is reserved for documented compartment syndrome confirmed by compartment pressure measurement, which is not established by swelling alone.

• B. Tourniquets can worsen local tissue necrosis and are not recommended in current snakebite management.
• C. Incision and suction are outdated, ineffective, and can increase local tissue damage and infection risk.
• D. Observation alone is inappropriate given progressive swelling and significant coagulopathy, which are indications for antivenom.
• E. Fasciotomy is reserved for confirmed compartment syndrome by pressure measurement, not for swelling alone, which is often from superficial venom effects.

Black widow (Latrodectus) venom contains alpha-latrotoxin, which causes massive presynaptic release of acetylcholine and norepinephrine at the neuromuscular junction and sympathetic terminals. This produces the classic syndrome of severe muscle pain, cramping, board-like abdominal rigidity without true peritoneal signs, diaphoresis, hypertension, and tachycardia, typically following a minimally painful bite. Diagnosis is clinical, based on exposure history and this characteristic presentation. Treatment is supportive with opioid analgesics and benzodiazepines; antivenom is reserved for severe, refractory cases.

• A. Organophosphate toxicity causes cholinergic excess with miosis, salivation, bradycardia, and bronchorrhea, not hypertensive tachycardia.
• C. Brown recluse bites cause a painless-to-mildly-painful lesion that progresses to dermonecrosis, not systemic neuromuscular toxicity.
• D. Appendicitis would show localized right lower quadrant tenderness and possible imaging findings, not diffuse rigidity with normal ultrasound.
• E. Scorpion stings cause local pain and can cause autonomic/neuromuscular symptoms, but the described rigid abdomen with normal imaging and the puncture history better fit a spider bite.

For bites by a stray or unknown-status dog, cat, or ferret that cannot be observed or tested, CDC guidelines recommend starting postexposure prophylaxis immediately with both human rabies immune globulin (passive immunity at the wound site) and the rabies vaccine series (active immunity), because rabies is almost universally fatal once symptomatic. The 10-day observation period applies only to healthy domestic dogs, cats, or ferrets that are available for observation, not to animals that have escaped. Wound cleaning alone is insufficient when there is a bite with mucosal or dermal exposure to saliva.

• A. The 10-day observation strategy only applies to an available, healthy domestic animal that can be monitored, not an animal that has fled.
• B. Tetanus prophylaxis should be given but does not replace rabies PEP in a high-risk, unprovoked stray dog bite.
• D. Risk cannot be excluded without observation or testing, so prophylaxis should not be withheld for a bite from an unknown-status stray.
• E. Immune globulin is required in addition to vaccine for unvaccinated individuals with high-risk exposures to provide immediate passive protection.

The ABCDE primary survey mandates airway assessment and management before all other interventions, since airway obstruction kills faster than breathing, circulatory, or neurologic problems. This patient has gurgling respirations indicating blood or secretions obstructing the airway, so suctioning and securing a patent airway (with cervical spine precautions) must occur first. Only after the airway is secured should breathing, circulation (including hemorrhage control and IV access), and disability (neurologic status) be addressed in that order. Delaying airway management to address the femur bleeding or obtain IV access risks hypoxic injury or death.

• A. Neurologic assessment (disability) comes after airway, breathing, and circulation in the primary survey sequence.
• B. A pelvic binder addresses circulation/hemorrhage control and is not indicated here; it also comes after airway and breathing in sequence.
• D. Hemorrhage control from the femur is part of circulation management, which follows airway and breathing in priority.
• E. IV access addresses circulation, which is assessed only after airway and breathing are secured.

Tension pneumothorax is a clinical diagnosis based on respiratory distress, hypotension, distended neck veins, tracheal deviation away from the affected side, absent breath sounds, and hyperresonance; imaging must never delay treatment because the patient can arrest from obstructive shock within minutes. The immediate life-saving intervention is needle decompression (now often followed promptly by tube thoracostomy), performed in the second intercostal space at the midclavicular line or fourth/fifth intercostal space at the midaxillary line. Waiting for a chest radiograph or echocardiogram to confirm the diagnosis is inappropriate and dangerous in this unstable patient.

• A. Obtaining imaging before treatment delays a life-saving intervention in a clinically obvious tension pneumothorax.
• B. Echocardiography evaluates for tamponade, not pneumothorax, and would still delay urgent decompression.
• D. Intubation does not relieve the tension pneumothorax and chest radiograph delays definitive treatment.
• E. A formal chest tube can follow, but needle decompression should occur first as the fastest temporizing measure; ultrasound guidance adds unnecessary delay in a clinical emergency.

Epidural hematoma classically follows temporal bone trauma with injury to the middle meningeal artery, producing a lucid interval followed by rapid neurologic deterioration as arterial bleeding accumulates. CT shows a lens-shaped (biconvex) hyperdensity that does not cross suture lines because the dura is tightly adherent to the skull at these points. The dilated, nonreactive pupil on the side of the lesion reflects uncal herniation compressing the ipsilateral oculomotor nerve, while contralateral weakness results from compression of the corticospinal tract. Rapid neurosurgical evacuation is required to prevent brainstem herniation and death.

• B. Subarachnoid hemorrhage appears as hyperdensity within the sulci and basal cisterns, not as a focal biconvex collection.
• C. Diffuse axonal injury typically shows punctate hemorrhages at gray-white junctions on CT or MRI, without a discrete hematoma.
• D. Subdural hematoma produces a crescent-shaped collection that can cross suture lines, not a lens-shaped one confined within them.
• E. Contusions appear as patchy, irregular areas of hemorrhage within the brain parenchyma, not as a well-defined biconvex extra-axial collection.

Subdural hematomas result from tearing of the bridging veins that traverse the subdural space connecting the cerebral cortex to the dural venous sinuses, a mechanism particularly common in elderly patients and those on anticoagulation due to brain atrophy increasing venous stretch. Because the bleeding is venous and the dura is not tightly fixed across sutures, the resulting crescent-shaped collection can cross suture lines, unlike epidural hematomas. The slower venous bleeding explains the delayed and progressive onset of symptoms over days, as seen in this patient. Anticoagulation, as with this patient's warfarin use, increases both the risk and the volume of bleeding after even minor head trauma.

• B. Middle meningeal artery rupture produces an arterial, rapidly expanding epidural hematoma with a lucid interval, not a crescent-shaped subdural collection.
• C. Aneurysmal rupture causes subarachnoid hemorrhage with diffuse hyperdensity in the basal cisterns and sulci, not a focal crescent-shaped extra-axial collection.
• D. Subarachnoid cortical artery rupture causes hemorrhage within the subarachnoid space, not a subdural crescent-shaped collection.
• E. Axonal shearing causes diffuse axonal injury seen as punctate white matter hemorrhages, not a discrete crescent-shaped fluid collection.

This patient demonstrates Cushing triad (hypertension, bradycardia, irregular respirations), a sign of critically elevated intracranial pressure with impending herniation, further supported by a new dilated, nonreactive pupil indicating uncal herniation. Immediate management includes securing the airway with endotracheal intubation, brief hyperventilation to acutely lower PaCO2 and induce cerebral vasoconstriction, and administration of a hyperosmolar agent such as mannitol or hypertonic saline to reduce cerebral edema while arranging emergent neurosurgical evaluation and CT imaging. Lowering blood pressure in this setting would reduce cerebral perfusion pressure and worsen ischemia, since the hypertension is a physiologic response to maintain perfusion against elevated intracranial pressure. Lumbar puncture is contraindicated with suspected elevated intracranial pressure due to the risk of precipitating herniation.

• A. There is no indication of infection; antibiotics do not address the underlying elevated intracranial pressure and herniation.
• C. Observation without intervention delays treatment of impending herniation, which is a neurosurgical emergency.
• D. Lowering blood pressure would reduce cerebral perfusion pressure and worsen ischemia; the hypertension is a compensatory response to elevated intracranial pressure.
• E. Lumbar puncture is contraindicated in suspected elevated intracranial pressure because it can precipitate brain herniation.

This patient meets all low-risk criteria of both the NEXUS and Canadian C-spine rules for clinical clearance without imaging: he is alert, has no midline cervical tenderness, no focal neurologic deficit, no intoxication, no distracting injury, and demonstrates full painless range of motion of the neck. When all these criteria are satisfied, the clinical decision rules have been validated with very high sensitivity for excluding clinically significant cervical spine injury, making imaging unnecessary. Applying these criteria reduces unnecessary radiation exposure, cost, and emergency department length of stay without compromising safety. Imaging should be reserved for patients who fail to meet one or more low-risk criteria.

• A. Plain radiographs have been largely replaced by clinical decision rules in appropriately selected low-risk patients and are unnecessary here.
• B. Prolonged collar use and admission are unnecessary and uncomfortable for a patient who meets validated low-risk clinical criteria.
• C. CT imaging is unnecessary when a patient meets all NEXUS/Canadian C-spine low-risk criteria and would expose him to unneeded radiation.
• E. MRI is reserved for suspected ligamentous or cord injury, not indicated in a patient who meets clinical clearance criteria.

In a hemodynamically unstable trauma patient, bedside ultrasound using the Focused Assessment with Sonography for Trauma (FAST) protocol is the appropriate initial test because it can be performed rapidly at the bedside without moving the patient, identifying free intraperitoneal fluid consistent with hemorrhage within minutes. A positive FAST in an unstable patient with suspected intra-abdominal hemorrhage supports proceeding directly to emergent laparotomy without the delay of CT, which requires transport to a scanner and a period of hemodynamic stability to safely obtain. Diagnostic peritoneal lavage is an older, invasive technique that has largely been replaced by FAST due to speed and noninvasiveness. Plain radiographs lack the sensitivity to detect hemoperitoneum.

• B. Diagnostic peritoneal lavage is invasive and has largely been replaced by FAST as the faster, noninvasive initial test.
• C. CT requires patient transport and hemodynamic stability, making it unsafe and impractical in an unstable patient.
• D. Plain radiographs do not reliably detect hemoperitoneum or solid organ injury and would delay identification of the bleeding source.
• E. Proceeding to laparotomy without first confirming intra-abdominal hemorrhage risks missing an alternative source of instability and unnecessary surgical risk.

Nonoperative management is the preferred approach for splenic injuries in hemodynamically stable patients, regardless of injury grade, as long as there is no contrast blush, active extravasation, or evidence of ongoing bleeding such as falling hemoglobin or hemodynamic instability. This patient's vital signs are stable, hemoglobin is stable on repeat measurement, and imaging shows no active bleeding, making close observation with serial hemoglobin checks, bed rest, and monitoring in a setting capable of emergent intervention the appropriate strategy. Operative or angiographic intervention is reserved for patients who are hemodynamically unstable, have a contrast blush or active extravasation, or show a falling hemoglobin indicating ongoing hemorrhage. Nonoperative management preserves splenic function and avoids the morbidity of surgery when safe to do so.

• A. Angioembolization is indicated when there is a contrast blush or active extravasation, which is absent here.
• B. Splenectomy is reserved for hemodynamic instability or failure of nonoperative management, not for a stable patient without active bleeding.
• C. Splenorrhaphy is a surgical splenic repair reserved for operative cases, not appropriate for a stable patient managed nonoperatively.
• D. Partial splenectomy is a surgical intervention unnecessary in a stable patient without active bleeding on imaging.

This patient has multiple signs predicting rapid airway compromise: hoarseness, stridor, singed nasal hairs, soot in the oropharynx, and circumferential neck burns. Inhalational injury causes progressive edema that can obstruct the airway within hours even when oxygen saturation is initially normal. Early, controlled intubation before edema worsens is safer than waiting until airway obstruction becomes severe, when intubation may become difficult or impossible. Normal oxygen saturation does not exclude impending airway compromise.

• B. CT imaging would delay definitive airway management and is not appropriate when stridor and other high-risk signs are already present.
• C. Oxygen alone does not address impending mechanical airway obstruction from progressive edema.
• D. Nebulized epinephrine does not prevent progressive airway edema in significant inhalational injury with stridor.
• E. Delaying definitive airway management to obtain laryngoscopy risks sudden complete obstruction.

Intramuscular epinephrine is the first-line treatment for anaphylaxis and should be given immediately in any patient with signs of systemic allergic reaction involving hypotension, airway compromise, or respiratory distress. Epinephrine acts on alpha-1 receptors to reverse vasodilation and hypotension, and on beta-2 receptors to relieve bronchospasm and reduce mediator release from mast cells and basophils. Antihistamines and corticosteroids are adjunctive therapies that have a slower onset and do not reverse the life-threatening hypotension or airway obstruction. Delaying epinephrine in favor of these agents increases the risk of progression to cardiovascular collapse or respiratory arrest.

• B. H2 blockers like famotidine are adjunctive and have no role in acute reversal of anaphylactic shock.
• C. Albuterol may help bronchospasm but does not address hypotension and is not first-line monotherapy for anaphylaxis.
• D. Corticosteroids have a delayed onset of action over hours and are not effective for immediate reversal of anaphylactic shock.
• E. Diphenhydramine relieves urticaria and itching but does not reverse hypotension or airway obstruction and has delayed onset.

The rhythm described is ventricular fibrillation, a shockable rhythm. For shockable rhythms (ventricular fibrillation and pulseless ventricular tachycardia), immediate defibrillation is the priority intervention, as early defibrillation significantly improves survival by terminating the chaotic electrical activity and allowing an organized rhythm to resume. Chest compressions should be minimized only for the time needed to deliver the shock, then resumed immediately afterward. Epinephrine and antiarrhythmics such as amiodarone are given after the first shock, not before.

• A. Synchronized cardioversion is used for stable tachyarrhythmias with a pulse, not for pulseless ventricular fibrillation.
• B. Epinephrine is administered after defibrillation attempts in a shockable rhythm, not as the initial step.
• C. Amiodarone is given after initial defibrillation attempts have failed, not before the first shock.
• E. Rhythm reassessment after 2 minutes of CPR applies to non-shockable rhythms or after a shock has already been delivered, not as a substitute for immediate defibrillation.

In hyperkalemia causing life-threatening cardiac arrhythmia or arrest, intravenous calcium gluconate (or calcium chloride) should be given first because it rapidly stabilizes cardiac myocyte membranes and antagonizes the arrhythmogenic effects of hyperkalemia within minutes, although it does not lower serum potassium. Insulin with dextrose, sodium bicarbonate, and beta-agonists like albuterol shift potassium intracellularly but take longer to act and do not provide the immediate membrane-stabilizing effect needed during arrest. Calcium should therefore precede or accompany these potassium-lowering therapies in an unstable or arresting patient.

• A. Oral potassium binders act over hours and have no role in the acute management of cardiac arrest.
• C. Albuterol shifts potassium intracellularly but has a delayed onset compared with calcium and is not first-line in arrest.
• D. Sodium bicarbonate has a slower, less reliable onset and is not the first agent given in hyperkalemic arrest.
• E. Insulin with dextrose lowers serum potassium by shifting it intracellularly but acts more slowly and does not provide immediate membrane stabilization.

When a first-line benzodiazepine fails to terminate status epilepticus, the next step is a second-line intravenous antiepileptic agent such as fosphenytoin, valproic acid, or levetiracetam, rather than repeating or switching benzodiazepines. These second-line agents provide longer-acting seizure control through different mechanisms, such as sodium channel blockade (fosphenytoin) or modulation of synaptic vesicle release (levetiracetam), and are guideline-recommended after benzodiazepine failure. If seizures persist despite an adequate second-line agent, the patient is considered to have refractory status epilepticus, and continuous infusion anesthetic agents are then used.

• B. Diazepam is also a benzodiazepine; adding another drug from the same class after an adequate dose has failed is not the recommended next step.
• C. Phenobarbital is typically reserved for cases refractory to both benzodiazepines and a first second-line agent, not as the immediate next step.
• D. Repeating benzodiazepines after an adequate first dose has already failed does not add meaningful efficacy and risks respiratory depression without escalating therapy appropriately.
• E. General anesthesia is reserved for refractory status epilepticus after second-line agents have failed, not as the second step.