Beta Blockers: The USMLE Step 1 Guide

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: E. Hypertrophic cardiomyopathy is most often caused by autosomal dominant mutations in sarcomere protein genes, producing myocyte hypertrophy and disarray that predispose to fatal ventricular arrhythmias, the most common cause of sudden cardiac death in young athletes. In patients with dynamic left ventricular outflow tract obstruction, any maneuver that decreases ventricular preload or afterload, or increases contractility, brings the hypertrophied septum and the anteriorly displaced mitral valve leaflet closer together, worsening obstruction; the Valsalva maneuver's straining phase decreases venous return and does exactly this. Conversely, maneuvers or drugs that increase preload, increase afterload, or decrease contractility widen the outflow tract and improve rather than worsen the obstruction. This directionality is frequently confused with the maneuvers used to evaluate fixed-obstruction murmurs such as aortic stenosis, a classic distinction tested on Step 1.

Answer: C. A beta-blocker blunts the heart-rate response needed to reach a target during an exercise stress test, so the nurse clarifies with the provider whether it should be held (option C). Administering it as usual (A) may prevent the client from reaching the target heart rate and give a nondiagnostic test. Giving a double dose (B) is unsafe and further blunts the response. Canceling the test (D) is unnecessary, since the provider can decide whether to hold the drug. Clarifying the medication plan ensures a valid, safe test. The nurse follows the provider's instruction and documents the medications taken.