Antibiotic Spectrum of Coverage: Step 1 Guide

Answer: A. Methicillin resistance in Staphylococcus aureus is conferred by the mecA gene, carried on the SCCmec cassette, which encodes PBP2a, an alternative transpeptidase that continues catalyzing peptidoglycan cross-linking even when native penicillin-binding proteins are inhibited, because PBP2a's active site is refractory to essentially all beta-lactam antibiotics. Option B describes the mechanism relevant to methicillin-susceptible, penicillin-resistant staphylococci, not the mecA-mediated mechanism specific to MRSA. Option C describes an efflux-based mechanism relevant to other antibiotic classes. Option D (erm-mediated ribosomal methylation) confers macrolide-lincosamide-streptogramin B resistance, an unrelated pathway. Option E describes fluoroquinolone resistance, mechanistically distinct from beta-lactam resistance.

Answer: A. Congenital long QT syndrome results from ion channelopathies, most often loss-of-function mutations in genes encoding cardiac potassium channels (reducing repolarizing potassium current) or gain-of-function mutations in sodium channel genes (prolonging depolarizing sodium current), both of which prolong the ventricular action potential and the QT interval; this creates conditions favorable for early afterdepolarizations, which can trigger the polymorphic ventricular tachycardia known as torsades de pointes, particularly during adrenergic stimulation such as swimming. Option B describes a conduction-based reentry mechanism unrelated to the repolarization abnormality that defines LQTS. Option C describes an unrelated bradycardia-atrial fibrillation mechanism, not the ventricular arrhythmia in this vignette. Option D describes catecholaminergic polymorphic ventricular tachycardia, a distinct channelopathy with a normal QT interval, not the prolonged QT interval described here. Option E describes an unrelated autoimmune mechanism affecting mechanical rather than electrical cardiac function.

Answer: B. Diphtheria toxin, encoded by a gene (tox) carried on a lysogenic corynephage, is absorbed into the bloodstream and catalyzes ADP-ribosylation of diphthamide, a modified histidine residue on eukaryotic elongation factor 2, which halts ribosomal translocation and protein synthesis in host cells throughout the body, explaining systemic complications such as myocarditis and demyelinating peripheral neuropathy that occur distant from the localized pharyngeal infection. Option A incorrectly attributes systemic disease to bacteremic organ seeding, when C. diphtheriae typically remains localized to the pharynx while its toxin disseminates hematogenously. Option C incorrectly borrows the cAMP-based mechanism specific to cholera toxin, a mechanistically distinct exotoxin. Option D incorrectly substitutes an immune complex mechanism for the direct toxin-mediated protein synthesis inhibition that is actually responsible. Option E correctly notes phage lysogenization is required but incorrectly claims this genetic element has no bearing on toxin production, when in fact the phage-encoded tox gene is precisely what enables toxin production.