Microbiology for USMLE Step 1

A 22-year-old college student has a 3-week history of fatigue, sore throat and fever. Examination shows posterior cervical lymphadenopathy, exudative pharyngitis and splenomegaly. He is given amoxicillin and develops a diffuse maculopapular rash. Which cell does the causative organism primarily infect?

1. B lymphocytes, via CD21 correct Correct. Epstein–Barr virus enters B cells through CD21 (complement receptor CR2). The atypical lymphocytes on the smear are reactive CYTOTOXIC T CELLS responding to those infected B cells, a distinction examiners test relentlessly.
2. Hepatocytes EBV commonly causes a mild transaminitis, but hepatocytes are not the primary target cell.
3. Respiratory epithelial cells only EBV does replicate in oropharyngeal epithelium, but the defining tropism and the reason for the lymphoproliferative picture is B cell infection.
4. CD4+ T lymphocytes, via gp120 That is HIV. Acute retroviral syndrome can mimic mononucleosis, but it would not produce the amoxicillin rash.
5. Neutrophils Not an EBV reservoir. The hematologic picture in mononucleosis is lymphocytosis, not neutrophilia.

The point: EBV infects B cells via CD21; atypical lymphocytes are reactive T cells. Ampicillin or amoxicillin rash is close to pathognomonic. Avoid contact sports for at least three weeks because of splenic rupture risk.

Source: USMLE Content Outline — microbiology NBME / FSMB (USMLE programme) · tier 0, exam blueprint / regulator

A 34-year-old woman is treated with ceftriaxone for pyelonephritis. Cultures grow a Gram-negative rod that is resistant to ceftriaxone and cefotaxime but susceptible to meropenem. The organism produces an enzyme that hydrolyzes the beta-lactam ring of extended-spectrum cephalosporins, and this enzyme is inhibited by clavulanic acid. Which resistance mechanism is described?

1. Efflux pump upregulation Actively exports the drug. Again, not enzymatic and not inhibited by clavulanate.
2. Carbapenemase production Would confer meropenem resistance, which the stem excludes. Carbapenemases such as KPC and NDM are the step beyond ESBL.
3. Porin channel loss reducing drug entry A real mechanism in Pseudomonas and Acinetobacter, but it reduces entry of many agents non-selectively and is not reversed by clavulanate.
4. Extended-spectrum beta-lactamase production correct Correct. ESBLs hydrolyze penicillins and extended-spectrum cephalosporins, are inhibited by clavulanate, and are typically plasmid-encoded in Escherichia coli and Klebsiella. Carbapenems remain the reliable treatment.
5. Alteration of penicillin-binding proteins The mechanism in MRSA (mecA producing PBP2a) and in penicillin-resistant pneumococcus. It is not inhibited by clavulanate, because no enzyme is being blocked.

The point: Beta-lactam resistance sorts into four mechanisms: enzymatic destruction (beta-lactamase, ESBL, carbapenemase, AmpC), altered target (PBP changes in MRSA), reduced entry (porin loss), and efflux. The clavulanate clue points specifically at a serine beta-lactamase.

Source: USMLE Content Outline — microbiology and antimicrobial resistance NBME / FSMB (USMLE programme) · tier 0, exam blueprint / regulator

A 58-year-old smoker develops pneumonia with high fever, confusion, diarrhea and a sodium of 128 mEq/L. Sputum Gram stain shows neutrophils but no organisms. He had recently stayed in a hotel with an air-conditioning fault. Which test confirms the diagnosis fastest?

1. Silver stain of induced sputum Used for Pneumocystis jirovecii, which presents with a dry cough and marked hypoxia in an immunocompromised patient.
2. Routine blood agar culture Legionella will not grow on standard media; it requires buffered charcoal yeast extract supplemented with cysteine and iron.
3. Cold agglutinin titre Points to Mycoplasma pneumoniae, a walking pneumonia in a younger patient without the hyponatremia and diarrhea.
4. Urinary antigen test correct Correct. Legionella pneumophila serogroup 1 is detected by urinary antigen within hours, and the organism is poorly seen on Gram stain because it stains faintly as a weak Gram-negative rod. Culture needs charcoal yeast extract with cysteine and iron.
5. Acid-fast stain For mycobacteria and Nocardia, neither of which fits an acute presentation over days with diarrhea and hyponatremia.

The point: Legionella triad worth memorising: pneumonia + diarrhea + hyponatremia, often with a raised transaminases and a relative bradycardia. Treat with a macrolide or a respiratory fluoroquinolone. Beta-lactams do not work, because it is intracellular.

Source: USMLE Content Outline — microbiology NBME / FSMB (USMLE programme) · tier 0, exam blueprint / regulator