USMLE (United States Medical Licensing Examination) Practice Exam 21
Practice exam for USMLE (United States Medical Licensing Examination).
Question 1
MEDIUMA 23-year-old man is brought to the emergency department after collapsing during a basketball game. He is alert but anxious. Blood pressure is 92/58 mm Hg, pulse is 168/min and irregular, and respirations are 18/min. ECG shows an irregularly irregular wide-complex tachycardia with varying QRS morphologies. He reports intermittent palpitations since adolescence. Which of the following best explains the mechanism responsible for this rhythm?
The vignette describes preexcited atrial fibrillation (irregularly irregular wide-complex tachycardia with varying QRS morphologies) in a young patient with a history of palpitations, consistent with Wolff-Parkinson-White syndrome. The mechanism is an accessory AV pathway (bundle of Kent) that bypasses the AV node, allowing rapid conduction to the ventricles during atrial fibrillation and producing wide, variable QRS complexes. AVNRT (dual AV nodal pathways) typically causes a regular narrow-complex tachycardia. Digoxin toxicity can cause atrial tachyarrhythmias with AV block but does not best fit this presentation and age/history.
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