USMLE Step 2 CK infectious diseases questions

For a previously healthy outpatient with community-acquired pneumonia and no comorbidities, the ATS/IDSA 2019 guideline recommends monotherapy with amoxicillin as first-line therapy because it provides reliable coverage against Streptococcus pneumoniae, the most common bacterial cause, while limiting unnecessary broad-spectrum exposure. Doxycycline or a macrolide are alternatives if there is penicillin allergy or intolerance. Respiratory fluoroquinolones and combination intravenous regimens are reserved for patients with comorbidities or those requiring hospitalization.

• A. This combination is appropriate for inpatient, non-ICU treatment, not for a healthy outpatient.
• B. This regimen targets pseudomonas and MRSA and is reserved for severe or healthcare-associated infections, not uncomplicated outpatient CAP.
• D. Doxycycline is an acceptable alternative but not the preferred first-line agent when there is no penicillin allergy.
• E. Respiratory fluoroquinolones are reserved for patients with comorbidities or risk factors for resistant organisms, not healthy outpatients.

Mycoplasma pneumoniae is a common cause of atypical pneumonia in young adults, especially in close-contact settings such as dormitories and military barracks, and classically presents with a dry cough, low-grade fever, and chest x-ray findings that look disproportionately severe compared to the relatively mild clinical presentation (a hallmark described as "walking pneumonia"). The normal white blood cell count and interstitial pattern rather than lobar consolidation further support an atypical organism. Diagnosis is typically clinical, supported by PCR testing if confirmation is needed, and treatment is with a macrolide or doxycycline since Mycoplasma lacks a cell wall and beta-lactams are ineffective.

• B. Streptococcus pneumoniae typically causes lobar consolidation with a more acute, toxic presentation and higher white blood cell count, not this mild interstitial pattern.
• C. Klebsiella pneumonia typically occurs in alcoholics or diabetics and causes a necrotizing lobar pneumonia with thick, bloody sputum, not this mild interstitial picture.
• D. Pneumocystis pneumonia occurs in immunocompromised hosts and this patient has no risk factors such as HIV infection or immunosuppressive therapy.
• E. Pulmonary embolism would present with acute dyspnea and pleuritic pain rather than a subacute illness with fever and infiltrates in a cluster of similarly affected contacts.

This patient's exposure to a hotel fountain and spa (aerosolized water sources), combined with gastrointestinal symptoms, hyponatremia, and mildly elevated liver enzymes accompanying pneumonia, is classic for Legionella pneumophila infection. The urinary antigen test is the best initial diagnostic test because it is rapid, highly specific, and detects Legionella serogroup 1, which accounts for most clinical disease, even after antibiotics have been started. Gram stain is not useful because Legionella is a small, poorly staining gram-negative organism that requires special culture media (buffered charcoal yeast extract agar) rather than standard blood culture techniques.

• A. Beta-D-glucan is used to detect invasive fungal infections such as Pneumocystis or Candida, not Legionella.
• B. Legionella does not grow on standard aerobic blood culture media; it requires specialized charcoal yeast extract agar.
• C. Cold agglutinins are associated with Mycoplasma pneumoniae infection, not Legionella.
• E. Legionella does not stain well with Gram stain and requires special culture media, so this is not useful for diagnosis.

This patient has aspiration pneumonia with evidence of early necrosis/cavitation, a risk in patients with impaired airway protection from alcohol intoxication and esophageal dysmotility. Ampicillin-sulbactam provides coverage against the typical oral anaerobes (such as Peptostreptococcus, Fusobacterium, and Prevotella) as well as aerobic gram-positive and gram-negative organisms that cause aspiration pneumonia and lung abscess, and is a guideline-recommended beta-lactam/beta-lactamase inhibitor combination for this indication. Alternatives such as clindamycin or a carbapenem can be used in penicillin-allergic patients, but macrolide, sulfonamide, or monotherapy aztreonam regimens lack adequate anaerobic coverage.

• A. Trimethoprim-sulfamethoxazole does not provide adequate coverage of anaerobic oral flora implicated in aspiration pneumonia.
• B. Doxycycline alone has unreliable anaerobic activity and is not a preferred choice for aspiration pneumonia with abscess.
• C. Azithromycin alone lacks reliable anaerobic coverage needed for aspiration pneumonia with abscess formation.
• E. Aztreonam covers gram-negative aerobes only and provides no anaerobic coverage, which is essential in this setting.

A cavitary lesion with an air-fluid level on chest x-ray, combined with a subacute course of fever, weight loss, and foul-smelling (putrid) sputum in a patient with poor dentition and alcohol use (risk factors for aspiration of anaerobic oral flora), is classic for a lung abscess. Putrid-smelling sputum strongly suggests anaerobic bacterial involvement, which is characteristic of abscesses arising from aspiration rather than other cavitary processes. Treatment typically involves prolonged antibiotics with anaerobic coverage, and most abscesses resolve without surgical drainage.

• A. Tuberculosis typically causes upper lobe cavitary disease without foul-smelling sputum and is usually associated with night sweats over a more chronic, often months-long course.
• C. Bronchogenic carcinoma can cavitate but would not typically produce foul-smelling purulent sputum, which points to an anaerobic infectious process.
• D. Empyema refers to pus within the pleural space, not a parenchymal cavity with an air-fluid level.
• E. Hydatid cysts are caused by Echinococcus and are rare in the absence of relevant exposure history such as travel to endemic sheep-raising regions.

In a patient with a clinical and radiographic presentation suspicious for active pulmonary tuberculosis, sputum acid-fast bacilli (AFB) smear and culture is the best initial test because it can rapidly identify mycobacteria on smear for infection control purposes and culture remains the gold standard for confirming active disease and guiding drug-susceptibility testing. Interferon-gamma release assays and tuberculin skin tests detect immune sensitization to M. tuberculosis but cannot distinguish latent from active infection and are not useful for diagnosing active disease in a symptomatic patient with cavitary lesions. Nucleic acid amplification testing is often performed alongside smear and culture, but sputum AFB smear/culture remains the foundational initial test.

• A. Bronchoscopy is invasive and reserved for cases where sputum cannot be obtained or initial testing is nondiagnostic.
• B. CT imaging can further characterize the lesion but does not provide microbiologic confirmation of active tuberculosis.
• D. This test also only indicates prior exposure or infection and does not confirm active disease in a symptomatic patient.
• E. This test detects immune sensitization and cannot distinguish latent infection from active tuberculosis disease.

Asymptomatic transaminase elevation less than 3-5 times the upper limit of normal in a patient without symptoms of hepatitis does not require stopping first-line anti-TB therapy. Current guidelines recommend continuing the regimen with close monitoring (repeat LFTs within 1-2 weeks) because mild enzyme elevation is common early in treatment and often self-resolves. Therapy should be interrupted only if AST/ALT exceed 3 times the upper limit of normal with symptoms (nausea, vomiting, jaundice, abdominal pain) or exceed 5 times the upper limit of normal regardless of symptoms. Isoniazid, rifampin, and pyrazinamide are all potentially hepatotoxic, so single-drug discontinuation is not indicated at this enzyme level.

• A. N-acetylcysteine has no established role in preventing or treating anti-TB drug-induced hepatotoxicity.
• B. Switching to second-line injectable therapy is unwarranted for mild, asymptomatic transaminase elevation.
• C. Stopping therapy is not indicated because the patient is asymptomatic and enzymes are below the threshold requiring discontinuation.
• D. There is no indication to selectively stop one drug when enzyme elevation is below the threshold for any intervention.

A biphasic illness pattern, in which a patient with influenza initially improves and then develops a sudden recurrence of high fever, worsening respiratory symptoms, and a cavitary or necrotizing infiltrate, is classic for secondary bacterial pneumonia caused by Staphylococcus aureus, including methicillin-resistant strains. S. aureus produces toxins (such as Panton-Valentine leukocidin) that cause necrotizing pneumonia with cavitation and hemoptysis, a pattern less typical of the other organisms. Streptococcus pneumoniae is the most common overall cause of post-influenza bacterial pneumonia but typically produces lobar consolidation without cavitation. Empiric therapy for suspected post-influenza S. aureus pneumonia should include vancomycin or linezolid pending cultures.

• B. S. pneumoniae is the most common post-influenza bacterial pathogen overall but does not typically cause cavitary, necrotizing infiltrates.
• C. M. pneumoniae causes an atypical, gradual pneumonia and is not associated with post-influenza cavitary necrotizing disease.
• D. K. pneumoniae causes cavitary pneumonia typically in alcoholics or debilitated patients, not classically as a post-influenza superinfection.
• E. H. influenzae causes post-influenza pneumonia but is not classically associated with cavitation and hemoptysis.

Antiviral therapy should be initiated in patients with confirmed or suspected influenza who are at high risk for complications, regardless of the 48-hour window typically cited for maximal benefit in healthy outpatients. This patient has diabetes mellitus and COPD, both recognized high-risk comorbidities per CDC guidance, so oseltamivir should be started as soon as possible even though she presents at 60 hours, because benefit still occurs beyond 48 hours in high-risk patients and treatment also reduces complications and duration of illness. She does not have signs of severe pneumonia or respiratory failure requiring hospitalization, so outpatient oral therapy is appropriate. Imaging and antibacterial therapy are not indicated without evidence of bacterial superinfection.

• A. She has no clinical findings suggesting pneumonia or complications that would require imaging before starting therapy.
• B. The 48-hour cutoff applies to maximizing benefit in low-risk patients; high-risk patients should still receive antivirals beyond 48 hours.
• C. There is no evidence of bacterial infection, and antivirals should not be withheld in a high-risk patient with confirmed influenza.
• E. She is hemodynamically stable with normal oxygenation and does not require hospitalization or intravenous therapy.

Prolonged paroxysmal cough with post-tussive vomiting and inspiratory whoop in an adult, following a catarrhal phase of mild upper respiratory symptoms, is characteristic of pertussis. Nasopharyngeal PCR is the preferred diagnostic test in the paroxysmal phase because it has higher sensitivity than culture, especially beyond the first 2 weeks of illness when organism burden declines, and results return faster than culture. Confirmation guides both treatment with a macrolide (such as azithromycin) and public health reporting, since pertussis is a reportable disease requiring contact prophylaxis. Sputum culture and Gram stain are insensitive for Bordetella pertussis, and chest imaging and bronchodilators do not address the underlying diagnosis.

• A. Chest CT is unnecessary when the chest x-ray is normal and the clinical picture is classic for pertussis.
• B. Albuterol does not treat the underlying infection and this presentation requires specific microbiologic diagnosis.
• C. Amoxicillin-clavulanate is not active against Bordetella pertussis; macrolides are the treatment of choice, but diagnosis should be confirmed first.
• E. Sputum Gram stain and culture have poor sensitivity for Bordetella pertussis and are not the preferred diagnostic method.

This patient has a classic presentation of acute bacterial meningitis with a petechial rash, raising strong suspicion for Neisseria meningitidis. Empiric antibiotic therapy should never be delayed for imaging, lumbar puncture, or culture results in a patient with suspected bacterial meningitis, because delay increases mortality and neurologic morbidity. Blood cultures should be drawn immediately before antibiotics, which was already done, and empiric therapy with vancomycin plus a third-generation cephalosporin (ceftriaxone) should be started right away to cover the most likely pathogens (S. pneumoniae and N. meningitidis), along with dexamethasone to reduce inflammatory complications, particularly for S. pneumoniae. Lumbar puncture can be performed as soon as feasible but should never delay empiric treatment.

• A. Oral doxycycline is inadequate empiric therapy for suspected bacterial meningitis and observation risks clinical deterioration.
• B. Antibiotics should be started immediately after blood cultures are drawn, not delayed until culture results return, which takes days.
• C. Antibiotics must never be delayed for lumbar puncture results in suspected bacterial meningitis due to the risk of rapid deterioration and death.
• E. Acyclovir alone does not cover the likely bacterial pathogens suggested by this presentation and rash.

This patient has several indications for head CT before lumbar puncture: immunocompromised status (HIV with CD4 90 cells/mm³), a new focal neurologic deficit (facial droop and arm weakness), and signs of papilledema on funduscopic exam suggesting raised intracranial pressure. Performing lumbar puncture in the presence of a mass lesion or significantly elevated intracranial pressure risks brain herniation, so CT should be obtained first to exclude a mass lesion or significant edema. Importantly, empiric antibiotics (and antivirals/antifungals as indicated) should still be started immediately, before and regardless of imaging results, so imaging does not delay treatment. Once CT excludes a contraindication, lumbar puncture can proceed safely.

• B. Spine MRI does not address the intracranial process suggested by headache, confusion, focal deficit, and papilledema.
• C. This presentation requires urgent inpatient evaluation and empiric treatment, not discharge home.
• D. Immediate lumbar puncture without imaging is unsafe here because of focal neurologic deficits and papilledema, which raise concern for a mass lesion or elevated intracranial pressure.
• E. EEG is not indicated as the next step in a patient with suspected CNS infection and focal deficits; CT to assess for safety of LP takes priority.

Group B Streptococcus (Streptococcus agalactiae) is the most common cause of neonatal meningitis, acquired by vertical transmission from maternal vaginal-rectal colonization during delivery. It appears on Gram stain as gram-positive cocci in chains, distinguishing it from the gram-negative rods of E. coli or the gram-positive rods of Listeria. Late-onset GBS disease (7-90 days of age) often presents with meningitis, while early-onset disease (under 7 days) more commonly presents as sepsis or pneumonia. CSF findings here show a bacterial profile with neutrophilic pleocytosis, low glucose, and elevated protein, consistent with bacterial meningitis.

• A. Streptococcus pneumoniae is a rare cause of meningitis in this age group and is not acquired via vertical transmission.
• B. Neisseria meningitidis is a gram-negative diplococcus and is an uncommon cause of meningitis in neonates.
• D. Listeria monocytogenes is a gram-positive rod, not cocci in chains, and is a less common cause than GBS.
• E. Escherichia coli is a gram-negative rod and would not produce this Gram stain appearance.

Herpes simplex virus encephalitis classically presents with fever, altered mental status, and seizures, with MRI showing asymmetric temporal and insular lobe involvement and CSF showing a lymphocytic pleocytosis with normal to mildly elevated protein. Empiric intravenous acyclovir should be started immediately on clinical suspicion because delayed treatment is associated with significantly increased morbidity and mortality, and therapy should not be withheld pending confirmatory PCR results. CSF HSV PCR is the diagnostic test of choice but results take time, and acyclovir has a favorable safety profile that supports early empiric use. Treatment is typically continued for 14-21 days if HSV PCR confirms the diagnosis.

• A. Oral valacyclovir has inadequate central nervous system penetration for treatment of encephalitis; intravenous acyclovir is required.
• B. Ganciclovir is used for cytomegalovirus infections, not HSV encephalitis, and is not the empiric treatment of choice here.
• D. Treatment should never be delayed pending PCR results given the high mortality of untreated HSV encephalitis.
• E. Vancomycin and ceftriaxone treat bacterial meningitis, which is not supported by the CSF profile or MRI findings here.

Brain abscesses arising from contiguous infection, such as dental, sinus, or otitic sources, develop through direct extension of infection through emissary or diploic veins and adjacent bone, typically producing a single abscess near the primary source, as seen here in the frontal lobe following a dental infection. This contrasts with hematogenous spread, which classically produces multiple abscesses at the gray-white junction, often in the distribution of the middle cerebral artery, and is associated with sources like endocarditis or pulmonary arteriovenous malformations. The clinical scenario of a preceding dental infection with a single frontal lobe lesion points to direct contiguous extension rather than bloodborne seeding.

• A. Pulmonary arteriovenous malformations cause hematogenous seeding with multiple lesions, not a single contiguous abscess from a dental source.
• B. Toxoplasmosis occurs in immunocompromised patients, typically with multiple ring-enhancing lesions, and there is no history of HIV or immunosuppression here.
• D. Endocarditis-related hematogenous spread typically causes multiple abscesses rather than a single contiguous lesion and lacks supporting cardiac findings here.
• E. The trigeminal nerve is not a recognized route for spread of odontogenic infection to the brain.

Cryptococcal meningitis should be strongly suspected in an HIV-infected patient with a CD4 count below 100 cells/mm³ presenting with subacute headache, fever, elevated opening pressure, and a mild lymphocytic CSF pleocytosis. The cerebrospinal fluid cryptococcal antigen test has sensitivity and specificity greater than 95% and is the most accurate test for diagnosis, more reliable than India ink staining, which has lower sensitivity, especially in patients with low fungal burden. Elevated opening pressure is a hallmark finding and guides management, as repeated therapeutic lumbar punctures are often needed to reduce intracranial pressure. Definitive diagnosis is confirmed by a positive CSF cryptococcal antigen or by growth of Cryptococcus neoformans on fungal culture.

• A. Beta-D-glucan is a marker for invasive fungal infections such as Aspergillus or Candida, but Cryptococcus has a different cell wall and typically does not elevate this assay.
• B. The VDRL test is used to diagnose neurosyphilis, not cryptococcal meningitis.
• D. Gram stain does not reliably detect encapsulated yeast and is used to identify bacteria, not fungi like Cryptococcus.
• E. Bacterial culture would not detect a fungal organism and the CSF profile here is not consistent with typical bacterial meningitis.

This patient's presentation of pain out of proportion to examination findings, hemorrhagic bullae, skin anesthesia, crepitus, and hemodynamic instability with markedly elevated creatine kinase is highly suspicious for necrotizing fasciitis, a surgical emergency. Immediate surgical exploration and debridement is both diagnostic and therapeutic, and should not be delayed for imaging studies when clinical suspicion is high, since mortality increases significantly with each hour of delay. Antibiotics are an important adjunct but are not sufficient alone to control a necrotizing soft tissue infection, since the infection spreads rapidly along fascial planes with impaired tissue perfusion limiting antibiotic penetration. Broad-spectrum antibiotics, including coverage for anaerobes, streptococci, staphylococci including MRSA, and gram-negative organisms, should be given concurrently with urgent surgery.

• A. Antibiotics alone are inadequate for necrotizing fasciitis because surgical source control is required to remove necrotic tissue and halt disease progression.
• B. Conservative management with elevation and oral antibiotics is inadequate and would allow rapid progression of this life-threatening infection.
• C. Hyperbaric oxygen therapy is not a substitute for emergent surgical debridement and has not been shown to improve outcomes as primary therapy.
• E. Imaging would delay life-saving surgery and is not necessary when clinical findings strongly suggest necrotizing fasciitis.

Once necrotizing fasciitis is confirmed intraoperatively, as evidenced by grayish necrotic fascia, lack of bleeding, and murky "dishwater" fluid, immediate and radical surgical debridement of all necrotic and infected tissue is required, often necessitating repeat debridements over subsequent days to ensure complete source control. Antibiotics alone cannot clear necrotic, poorly perfused tissue and surgery remains the cornerstone of treatment, with delay directly correlating with increased mortality. Clindamycin is added to antibiotic regimens in necrotizing soft tissue infections for its antitoxin effect against toxin-producing strains such as group A Streptococcus. Supportive care with fluid resuscitation and vasopressor support as needed should proceed in parallel with surgery, not as an alternative to it.

• A. Topical dressings do not address the underlying necrotic tissue and infection and are not a substitute for surgical debridement.
• B. Delaying debridement allows continued spread of infection along fascial planes and increases mortality; surgery should not be delayed once the diagnosis is confirmed.
• C. Skin grafting is a reconstructive procedure performed after debridement and infection control, not before.
• E. Oral antibiotics have inadequate tissue penetration and are never appropriate for an active necrotizing soft tissue infection.

For a simple cutaneous abscess without surrounding cellulitis, systemic signs of infection, or features suggestive of a complicated infection, incision and drainage alone is the most appropriate and sufficient treatment, as this addresses the primary purulent collection and most small abscesses resolve without additional antibiotics. Adjunctive antibiotics such as trimethoprim-sulfamethoxazole or clindamycin are reserved for abscesses with systemic symptoms, extensive surrounding cellulitis, immunosuppression, abscess larger than 2 cm with severe disease, failed initial drainage, or in areas difficult to drain completely. Since this patient has no fever, no significant surrounding cellulitis, and no systemic illness, drainage alone is appropriate per IDSA guidelines for skin and soft tissue infections. Culture of the purulent material can help guide therapy if the patient fails to improve or recurs.

• A. Antibiotics without drainage are inadequate for a fluctuant abscess because source control through drainage is required to resolve the purulent collection.
• B. Warm compresses alone without drainage are insufficient for a fluctuant abscess that requires incision and drainage for resolution.
• D. Cephalexin does not reliably cover community-acquired MRSA and antibiotics without drainage are not appropriate management for a fluctuant abscess.
• E. Intravenous antibiotics and hospitalization are unnecessary for a simple, localized abscess without systemic signs of infection.

Disseminated gonococcal infection classically presents in young, sexually active patients with a migratory polyarthralgia or tenosynovitis phase followed by a purulent monoarthritis phase, accompanied by painless pustular or vesiculopustular skin lesions on an erythematous base. Synovial fluid cultures are often negative because the organism is present in low numbers and is fastidious, so cervical, urethral, rectal, or pharyngeal cultures (or nucleic acid amplification testing) are more sensitive for confirming Neisseria gonorrhoeae. The combination of tenosynovitis, dermatitis, and polyarthralgia is a classic triad for this diagnosis. Staphylococcal septic arthritis typically presents as an acute, monoarticular process with higher synovial leukocyte counts and a positive Gram stain or culture.

• A. Acute rheumatic fever follows streptococcal pharyngitis and presents with migratory large-joint arthritis plus carditis, chorea, or subcutaneous nodules, not pustular skin lesions.
• B. Lyme arthritis presents with large joint monoarthritis, often preceded by erythema migrans, and is not associated with tenosynovitis or pustular lesions.
• C. Staphylococcal septic arthritis usually causes a markedly higher synovial WBC count (>50,000/mm³) and a positive Gram stain, and does not typically present with tenosynovitis or pustular skin lesions.
• D. Reactive arthritis follows a GI or GU infection by 1-4 weeks and is typically associated with conjunctivitis or urethritis, not pustular skin lesions and tenosynovitis.

In any patient with an acutely hot, swollen joint and fever, septic arthritis must be excluded before starting antibiotics, and the single most important step is arthrocentesis with synovial fluid cell count, Gram stain, and culture. Fluid should be obtained prior to antibiotic administration whenever possible, since antibiotics can sterilize the fluid and obscure the diagnosis; if the patient is septic, empiric antibiotics can be started immediately after the aspiration is performed. A history of gout does not exclude septic arthritis, and the two conditions can coexist, so synovial fluid analysis is needed to evaluate for crystals and infection simultaneously.

• A. Colchicine would be inappropriate without first excluding joint infection, since treating septic arthritis as gout can lead to joint destruction and sepsis.
• B. MRI is not the first step in an acutely inflamed native joint when arthrocentesis can rapidly provide a diagnosis.
• C. Plain radiographs are low-yield in early septic arthritis and do not change the need for urgent arthrocentesis.
• D. Antibiotics should not be given before synovial fluid is obtained, as this can sterilize the joint fluid and prevent identification of the organism.

Vertebral osteomyelitis (often with discitis) classically shows increased T2 and decreased T1 signal in the intervertebral disc and adjacent vertebral endplates, with endplate destruction and post-contrast enhancement of the disc and adjacent vertebral bodies on MRI, which is the most sensitive and specific imaging modality. Intravenous drug use is a major risk factor for hematogenous seeding of the spine, and Staphylococcus aureus is the most common causative organism. The disc space is involved early because the infection typically spreads from one vertebral endplate across the disc to the adjacent vertebra, a pattern distinct from malignancy, which usually spares the disc.

• B. Epidural lipomatosis is associated with chronic steroid use or obesity and does not explain fever and elevated inflammatory markers from bacteremia.
• C. Isolated facet joint effusion without disc involvement is more typical of facet joint septic arthritis, which is less common than discitis-osteomyelitis in this risk group.
• D. Decreased T2 signal and preserved disc height are not expected; infection causes increased T2 signal and disc/endplate destruction.
• E. A compression fracture suggests trauma or malignancy, not the typical appearance of pyogenic vertebral osteomyelitis.

Cat bites carry a high risk of infection with Pasteurella multocida, a gram-negative organism, in addition to oral anaerobes and streptococcal and staphylococcal species. Amoxicillin-clavulanate provides coverage against Pasteurella, anaerobes, and gram-positive skin flora, making it the preferred prophylactic and treatment choice for cat and dog bites per IDSA guidelines. Prophylactic antibiotics are generally recommended for cat bites given their high infection rate due to deep puncture wounds that inoculate bacteria deeply into tissue.

• A. Clindamycin has poor activity against Pasteurella multocida and is not first-line for bite wound prophylaxis.
• B. Cephalexin lacks reliable activity against Pasteurella multocida and anaerobes.
• D. Trimethoprim-sulfamethoxazole has unreliable anaerobic coverage and is not first-line for bite wound prophylaxis.
• E. Doxycycline alone does not adequately cover anaerobes typically found in bite wounds, though it can be used in penicillin-allergic patients combined with other agents.

Intravenous drug use is the classic risk factor for right-sided (tricuspid valve) infective endocarditis caused by Staphylococcus aureus, which seeds directly from the venous circulation. Vegetations on the tricuspid valve fragment and embolize to the pulmonary arterial circulation, producing multiple bilateral, peripheral, often cavitary nodular infiltrates on imaging, a finding essentially pathognomonic for septic pulmonary emboli in this clinical context. This differs from left-sided endocarditis, where emboli travel systemically rather than to the lungs.

• A. Immune complex-mediated vasculitis is not associated with cavitary nodules and is more typical of glomerulonephritis in subacute endocarditis.
• B. Aspiration pneumonia typically produces a focal consolidation in a dependent lung segment, not multiple bilateral cavitary nodules.
• D. While skin infection can cause bacteremia, the multiple cavitary nodular pattern specifically reflects septic emboli from a cardiac source, not direct hematogenous spread without a nidus.
• E. There is no evidence of mediastinal infection or contiguous spread in this vignette.

In a hemodynamically stable patient with suspected subacute infective endocarditis, the most appropriate next step is to obtain at least three sets of blood cultures from separate venipuncture sites before initiating antibiotics, since this maximizes the yield for identifying the causative organism and guides targeted therapy. Antibiotics should generally be withheld until cultures are drawn unless the patient is septic or hemodynamically unstable, in which case empiric therapy is started immediately after cultures are obtained. Echocardiography (starting with transthoracic, reserving transesophageal for inadequate windows or suspected complications) is important for diagnosis but does not take priority over obtaining blood cultures first.

• A. Blood cultures should be obtained before imaging, since identifying the organism is the priority and does not require echocardiography first.
• B. Starting antibiotics before obtaining blood cultures in a stable patient can sterilize the blood and prevent identification of the causative organism.
• D. Empiric antibiotics should not be started before blood cultures are drawn in a stable patient, as this reduces culture yield and can delay identification of the causative organism.
• E. Transesophageal echocardiography is not first-line; transthoracic echocardiography is typically performed first, and blood cultures are drawn before any imaging.

This patient's occupational exposure to sheep, chronic constitutional symptoms, and elevated phase I IgG antibodies point to Coxiella burnetii (Q fever) endocarditis. C. burnetii is an obligate intracellular organism that cannot be isolated using standard blood culture techniques, making serology the diagnostic method of choice; a phase I IgG titer greater than 1:800 is highly suggestive of chronic Q fever endocarditis. HACEK organisms, by contrast, are fastidious Gram-negative bacilli that can eventually grow in culture with extended incubation. Culture-negative endocarditis should prompt consideration of Coxiella, Bartonella, and Tropheryma whipplei, which require serology, PCR, or special culture techniques rather than routine blood cultures.

• A. There is no mention of fungal risk factors, and the phase I antibody titer specifically indicates a different pathogen.
• B. This describes HACEK organisms, which are not obligate intracellular organisms and are not consistent with the sheep exposure and serology described.
• D. The serologic findings provide a specific diagnosis rather than suggesting a technical collection error.
• E. The vignette does not mention prior antibiotic use, and the specific serologic finding points to a defined causative organism.

For acute uncomplicated cystitis in a healthy, nonpregnant woman, first-line empiric agents are nitrofurantoin for 5 days, trimethoprim-sulfamethoxazole for 3 days (in areas with low local resistance), or fosfomycin as a single dose. Nitrofurantoin achieves high concentrations in the urine but poor tissue penetration, making it ideal for bladder-limited infection but unsuitable for pyelonephritis. Fluoroquinolones are effective but are reserved as alternative agents due to concerns about resistance and adverse effects, and should not be first-line for uncomplicated cystitis. Amoxicillin is not recommended empirically due to high rates of E. coli resistance.

• B. Doxycycline is not a recommended first-line agent for uncomplicated cystitis and lacks reliable activity against common uropathogens in this setting.
• C. Fluoroquinolones are effective but reserved as second-line therapy for uncomplicated cystitis due to resistance and safety concerns.
• D. Amoxicillin has high rates of E. coli resistance and is not recommended as empiric therapy for cystitis.
• E. Intravenous ceftriaxone is unnecessary and overly aggressive for uncomplicated outpatient cystitis.

This patient has pyelonephritis complicated by signs of sepsis, including hypotension, tachycardia, tachypnea, and elevated lactate, as well as inability to tolerate oral intake due to vomiting. These findings mandate inpatient admission for intravenous fluid resuscitation and intravenous antibiotics rather than outpatient oral therapy. Indications for hospitalization in pyelonephritis include hemodynamic instability, inability to maintain oral hydration or tolerate oral antibiotics, pregnancy, uncertain diagnosis, or failure of outpatient therapy. Uncomplicated pyelonephritis in a hemodynamically stable patient who can tolerate oral intake can be managed as an outpatient with oral fluoroquinolones.

• A. A single intramuscular dose is insufficient treatment and does not address this patient's hemodynamic instability.
• B. Outpatient oral therapy is inappropriate given hemodynamic instability and inability to tolerate oral intake.
• D. Brief observation does not address the need for fluid resuscitation and intravenous antibiotics in a septic patient.
• E. This patient cannot tolerate oral medications due to vomiting and has signs of sepsis requiring inpatient care.

Asymptomatic bacteriuria in pregnancy is treated because, unlike in nonpregnant women, it is associated with an increased risk of progression to pyelonephritis as well as adverse pregnancy outcomes such as preterm birth and low birth weight. All pregnant women should be screened with a urine culture in early pregnancy, and a positive culture with a single uropathogen at a colony count of 100,000 CFU/mL or greater should be treated with an appropriate oral antibiotic course even in the absence of symptoms. A follow-up test of cure culture is typically obtained after treatment completion. This differs from nonpregnant, non-catheterized patients, in whom asymptomatic bacteriuria is not treated.

• A. Unlike in nonpregnant women, asymptomatic bacteriuria in pregnancy is treated regardless of symptom development because of obstetric risk.
• B. Deferring treatment increases the risk of progression to pyelonephritis and adverse pregnancy outcomes; treatment should not be delayed.
• D. Prolonged prophylactic antibiotics for the remainder of pregnancy are not the standard approach to a single episode of asymptomatic bacteriuria.
• E. Imaging is not required before treating a confirmed positive urine culture in pregnancy.

In women with uncomplicated recurrent UTIs (generally defined as two or more infections in 6 months or three or more in a year), with no anatomic abnormality, hematuria, or atypical features, further urologic imaging workup is not routinely indicated. Appropriate management includes counseling on behavioral risk factors, non-antimicrobial preventive strategies, and antibiotic strategies such as continuous low-dose prophylaxis, postcoital prophylaxis, or patient-initiated self-treatment. Imaging or cystoscopy is reserved for patients with features suggesting an underlying structural or functional abnormality, such as hematuria, incomplete response to treatment, obstructive symptoms, or recurrent upper tract infection. This patient's presentation is typical and uncomplicated, so an extensive workup is unnecessary.

• A. Cystoscopy is reserved for patients with atypical features such as hematuria or suspected structural abnormality, which this patient does not have.
• C. CT urogram is not indicated in uncomplicated recurrent cystitis without red-flag findings.
• D. Antibiotic prophylaxis should be a shared decision after discussing behavioral and non-antibiotic options, not started indefinitely without discussion.
• E. Renal imaging and voiding studies are reserved for complicated cases or suspected anatomic abnormality, not typical recurrent uncomplicated cystitis.

The preferred diagnostic approach for suspected C. difficile infection in a symptomatic patient is a multistep algorithm: a highly sensitive nucleic acid amplification test (PCR) for toxin genes combined with a toxin enzyme immunoassay, because PCR alone can detect colonization without active toxin production and lead to overdiagnosis. A positive result on both tests confirms active toxin-producing infection in a symptomatic patient. Toxigenic culture is sensitive but slow and used mainly for epidemiologic purposes, not routine clinical diagnosis. The combination testing strategy balances sensitivity and specificity better than either test alone.

• B. Toxigenic culture is highly sensitive but too slow for routine clinical use and is mainly reserved for epidemiologic studies.
• C. Serum antibody testing is not a validated or standard method for diagnosing active C. difficile infection.
• D. Ova and parasite examination does not detect C. difficile and is not relevant to this clinical picture.
• E. Fecal lactoferrin and stool white cell count indicate inflammation but do not establish a specific diagnosis of C. difficile infection.

Antimotility agents such as loperamide are avoided in inflammatory or bloody diarrhea, particularly when Campylobacter, Salmonella, or Shigella is suspected or confirmed, because slowing gut transit can prolong mucosal contact with the organism or its toxins and has been associated with worse outcomes, including toxic megacolon and prolonged bacteremia risk. Supportive care with oral or IV rehydration is the mainstay; antibiotics are reserved for severe or high-risk cases. The organism described (curved, oxidase-positive gram-negative rod) is consistent with Campylobacter jejuni.

• B. Inflammatory, bloody diarrhea is a contraindication to antimotility agents, not an indication.
• C. Oral vancomycin is not used for Campylobacter infection; it is reserved for C. difficile.
• D. Bismuth subsalicylate does not create a beneficial synergy with loperamide in this setting and does not address the underlying concern.
• E. Routine antibiotics are not required for most immunocompetent patients with Campylobacter enteritis unless illness is severe or prolonged, and combining with loperamide is not recommended.

Spontaneous bacterial peritonitis (SBP) is diagnosed when ascitic fluid absolute neutrophil count is 250 cells/mm3 or higher in a patient with ascites, without a surgical source of infection. Empiric therapy is a third-generation cephalosporin, most commonly intravenous ceftriaxone, which covers the typical pathogens (E. coli, Klebsiella, Streptococcus pneumoniae) and achieves adequate ascitic fluid levels. Albumin is added in patients with significant renal impairment or bilirubin elevation to reduce risk of hepatorenal syndrome. Treatment should not be delayed pending further fluid analysis once the neutrophil threshold is met.

• B. Oral fluoroquinolones are used for SBP prophylaxis in selected patients, not as treatment for an acute episode with these findings.
• C. SBP is a medical, not surgical, diagnosis; laparotomy is unnecessary and risks harm absent signs of secondary peritonitis.
• D. The ANC of 650 cells/mm3 already exceeds the diagnostic threshold of 250 cells/mm3; treatment should begin immediately, not be delayed for repeat testing.
• E. Metronidazole plus ciprofloxacin is not the standard empiric regimen and unnecessarily broadens anaerobic coverage not typically needed in SBP.

Current CDC guidelines recommend empiric treatment of suspected gonococcal infection with a single intramuscular dose of ceftriaxone (500 mg for patients under 150 kg) plus oral doxycycline 100 mg twice daily for 7 days to cover concurrent chlamydial infection, since the two infections frequently coexist and clinical differentiation is unreliable. Azithromycin is no longer the preferred chlamydia regimen for most patients due to doxycycline's superior efficacy, particularly for rectal chlamydia. Dual empiric therapy should be given at the time of presentation in symptomatic patients at high risk rather than waiting for NAAT confirmation.

• B. Penicillin is not effective against Neisseria gonorrhoeae due to widespread resistance and is used for syphilis, not gonorrhea or chlamydia.
• C. Ceftriaxone alone does not address the likely coinfection with chlamydia.
• D. Azithromycin alone does not treat gonorrhea and is no longer first-line for chlamydia monotherapy in this context.
• E. Fluoroquinolone resistance in Neisseria gonorrhoeae is widespread, so ciprofloxacin is not recommended for empiric gonorrhea treatment.

This patient's fever, chills, myalgias, and rash flare within 24 hours of treatment for syphilis represent a Jarisch-Herxheimer reaction, caused by the release of lipopolysaccharide-like products and inflammatory cytokines as large numbers of treponemes are killed. It is self-limited, typically resolving within 24 hours, and is managed with antipyretics and supportive care without stopping the antibiotic. It is not a drug allergy and does not require switching antibiotics, skin testing, or epinephrine. Patients should be counseled about this possibility before treatment, especially in secondary syphilis where spirochete burden is high.

• A. An extra dose of penicillin is not needed and does not address the underlying cytokine-mediated reaction.
• B. Epinephrine is used for anaphylaxis, which this is not; there is no urticaria, angioedema, or hemodynamic collapse.
• C. Skin testing evaluates IgE-mediated penicillin allergy, which is not the mechanism of this self-limited reaction.
• E. The reaction is not an allergic response to penicillin, so switching antibiotics is unnecessary and penicillin remains first-line.

Penicillin is the only antibiotic with proven efficacy for preventing congenital syphilis, and no alternative regimen is considered reliably effective in pregnancy. Pregnant women with a confirmed penicillin allergy, especially a history consistent with IgE-mediated anaphylaxis, should undergo penicillin desensitization in a controlled setting and then receive the appropriate penicillin regimen for their stage of syphilis. Doxycycline is contraindicated in pregnancy and not recommended for treating maternal syphilis, and ceftriaxone and azithromycin have inadequate data to ensure fetal treatment. Delaying treatment risks transplacental transmission and congenital syphilis.

• A. Azithromycin has documented treponemal resistance and is not recommended for treating syphilis in pregnancy.
• B. Ceftriaxone has not been validated as reliably effective for treating syphilis in pregnancy and is not recommended as an alternative.
• C. Delaying maternal treatment risks ongoing transplacental infection and congenital syphilis; treatment should not be postponed.
• D. Doxycycline is contraindicated in pregnancy due to effects on fetal bone and teeth and is not used to treat maternal syphilis.

Nucleic acid amplification testing (PCR) of a swab from the base of an unroofed ulcer is the most sensitive and specific method to confirm genital herpes simplex virus infection and has largely replaced viral culture as the preferred test. It also allows typing as HSV-1 or HSV-2, which has prognostic value for recurrence risk. Given this patient's recurrent episodic pattern, after confirmation she could be offered either episodic therapy with oral valacyclovir at symptom onset or daily suppressive therapy with valacyclovir if recurrences are frequent or bothersome. Serologic IgG testing can indicate past exposure but does not confirm the cause of a current lesion.

• A. Darkfield microscopy identifies spirochetes of syphilis, not herpes simplex virus.
• B. Serum IgG testing indicates past exposure and cannot confirm that the current ulcer is due to active HSV infection.
• D. Tzanck smear has low sensitivity and specificity compared to PCR and is no longer favored for confirming HSV.
• E. RPR is a screening test for syphilis, not herpes simplex virus.

Chancroid, caused by Haemophilus ducreyi, classically presents as one or more painful ulcers with ragged, undermined borders and a friable, necrotic base, accompanied by unilateral tender inguinal lymphadenitis that can progress to fluctuant buboes. The 'school of fish' pattern of gram-negative rods on Gram stain is a classic, though insensitive, finding. Lymphogranuloma venereum produces a small painless ulcer with prominent, often matted lymphadenopathy, granuloma inguinale produces a painless, beefy-red, friable ulcer without significant lymphadenopathy, and syphilis and herpes ulcers are typically painless or vesicular rather than deeply ragged and necrotic.

• A. Primary syphilis presents with a painless, indurated ulcer (chancre), not a painful ragged necrotic ulcer.
• C. Lymphogranuloma venereum typically causes a transient painless ulcer followed by prominent painful lymphadenopathy, not a deep painful necrotic ulcer with a positive Gram stain for this organism.
• D. Genital herpes presents with painful grouped vesicles or shallow ulcers, not a single deep ulcer with a necrotic base and gram-negative rods on Gram stain.
• E. Granuloma inguinale causes a painless, beefy-red, friable ulcer without significant regional lymphadenopathy, unlike this patient's painful ulcer and tender bubo.

The CDC-recommended algorithm begins with a fourth-generation combination immunoassay that detects both HIV p24 antigen and HIV-1/HIV-2 antibodies, which shortens the window period compared to antibody-only tests by detecting antigen before seroconversion. A reactive result is followed by an HIV-1/HIV-2 antibody differentiation immunoassay to confirm and distinguish HIV-1 from HIV-2; if that result is indeterminate or negative despite a reactive screening test, an HIV-1 RNA test is used to resolve acute infection. Western blot is no longer part of the recommended confirmatory algorithm. CD4 count is used for staging and management, not diagnosis.

• A. Western blot has been removed from the current CDC-recommended testing algorithm due to lower sensitivity during early infection.
• B. RNA testing is reserved for resolving discordant or indeterminate results after screening and supplemental antibody testing, not as the initial screen.
• D. CD4 count assesses immune status in known HIV infection and does not diagnose the infection.
• E. The differentiation immunoassay is the confirmatory, not initial, test used after a reactive fourth-generation screening result.

This presentation is classic for acute retroviral syndrome, which typically occurs 2 to 4 weeks after HIV exposure with fever, pharyngitis, lymphadenopathy, myalgias, and a diffuse rash resembling infectious mononucleosis. During this very early window, antibody production may be insufficient for a fourth-generation test to turn positive, even though it also detects p24 antigen, because viral replication may still precede detectable antigen or antibody levels in some individuals. An HIV-1 RNA viral load test detects viremia before seroconversion and is the most sensitive test to diagnose acute HIV infection when a combination immunoassay is negative but clinical suspicion is high.

• A. CD4 count can be low transiently in acute infection but is not the most sensitive or specific test to diagnose acute HIV infection.
• B. Antiretroviral therapy should not be started until the diagnosis is confirmed, though it will be initiated promptly once confirmed.
• C. A negative fourth-generation test cannot exclude very early acute HIV infection when clinical suspicion is high.
• E. Waiting 3 months delays diagnosis and appropriate counseling and treatment; a more sensitive test should be obtained now.

Current guidelines recommend initiating antiretroviral therapy in all persons with HIV as soon as possible after diagnosis, regardless of CD4 count or viral load, because early treatment reduces morbidity, mortality, and risk of transmission. A preferred first-line regimen is an integrase strand transfer inhibitor combined with two nucleoside reverse transcriptase inhibitors, such as the single-tablet regimen bictegravir/emtricitabine/tenofovir alafenamide, which offers high efficacy, a high barrier to resistance, and a favorable tolerability profile. Efavirenz-based regimens are no longer preferred first-line due to central nervous system side effects, and antiretroviral therapy should never be given as monotherapy due to the rapid development of resistance.

• A. Treatment should not be deferred based on viral load threshold; therapy is recommended for all patients with HIV at diagnosis.
• C. Efavirenz-based regimens are associated with significant neuropsychiatric side effects and are no longer a preferred first-line option.
• D. Current guidelines recommend starting antiretroviral therapy immediately after diagnosis regardless of CD4 count, not waiting for it to decline.
• E. Monotherapy is never appropriate for HIV treatment because it leads to rapid selection of resistant viral strains.

Pneumocystis jirovecii pneumonia prophylaxis is indicated when the CD4 count falls below 200 cells/mm3, and Toxoplasma gondii prophylaxis is added when the CD4 count falls below 100 cells/mm3 in a patient who is Toxoplasma IgG-seropositive. Trimethoprim-sulfamethoxazole is the preferred agent for both indications because it provides effective prophylaxis against both organisms simultaneously, simplifying the regimen for a patient who meets both thresholds. Mycobacterium avium complex prophylaxis is no longer routinely recommended for patients on effective antiretroviral therapy, even at very low CD4 counts, given the effectiveness of modern ART and the low incidence of disseminated MAC when viral suppression is achieved.

• A. Dapsone can be used as an alternative for Pneumocystis prophylaxis in sulfa-allergic patients but does not adequately cover Toxoplasma prophylaxis alone at standard dosing.
• B. Routine primary MAC prophylaxis is no longer recommended in patients who are on antiretroviral therapy, and azithromycin alone would not cover this patient's Pneumocystis and Toxoplasma risk.
• D. This patient's CD4 count of 85 cells/mm3 is below both the Pneumocystis and Toxoplasma prophylaxis thresholds, so prophylaxis is indicated.
• E. Routine MAC prophylaxis is not recommended for patients receiving antiretroviral therapy regardless of CD4 count.

This patient meets criteria for septic shock from presumed urosepsis (hypotension, elevated lactate, altered mentation). Current Surviving Sepsis Campaign guidelines recommend immediate administration of 30 mL/kg of intravenous crystalloid for hypoperfusion and initiation of empiric broad-spectrum antibiotics within 1 hour of recognition, after obtaining cultures. Delaying resuscitation or antibiotics for imaging increases mortality. Vasopressors are added if hypotension persists despite adequate fluid resuscitation, not as an initial substitute for fluids.

• A. Vasopressors are indicated only if hypotension persists after adequate fluid resuscitation, not as first-line therapy.
• B. Imaging should not delay life-saving fluid resuscitation and antibiotics in septic shock.
• C. Hydrocortisone is reserved for septic shock refractory to fluids and vasopressors, not as an initial step.
• E. Empiric antibiotics should be started immediately in septic shock, not delayed for culture results.

This patient has febrile neutropenia (absolute neutrophil count <500/mm³ with fever) and requires immediate empiric monotherapy with an antipseudomonal beta-lactam such as cefepime, piperacillin-tazobactam, or a carbapenem, since Pseudomonas aeruginosa infection can be rapidly fatal in neutropenic patients. Vancomycin is added only for specific indications such as hemodynamic instability, suspected catheter-related infection, skin/soft tissue infection, or known MRSA colonization, not as initial monotherapy. Oral regimens are reserved for low-risk patients who meet criteria for outpatient management, which does not apply here given her high-risk features (prolonged neutropenia, mucositis).

• A. Oral therapy is reserved for low-risk febrile neutropenia patients who can be managed as outpatients; she has high-risk features requiring intravenous antipseudomonal coverage.
• B. Vancomycin alone does not cover gram-negative organisms including Pseudomonas and should only be added to, not substituted for, antipseudomonal beta-lactam therapy.
• C. Fluconazole targets fungal infection and is not appropriate empiric first-line therapy for initial febrile neutropenia without signs of fungal disease.
• D. Trimethoprim-sulfamethoxazole does not provide adequate antipseudomonal coverage required for empiric therapy in febrile neutropenia.