If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 3 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
This month’s stumper
A 45-year-old man is brought to the emergency department after fainting while walking his dog. He denies a history of chest pain or prior syncope. His previous medical history is significant for a cardiac murmur known since childhood. He does not smoke and denies any recreational drug use.
His temperature is 37 °C (98.6 °F), blood pressure is 128/78 mm Hg, pulse is 87 beats per minute and regular, and respiratory rate is 18 per minute. He has a prominent apical impulse and II/VI, late peaking murmur at the right upper sternal border that decreases in intensity with a Valsalva maneuver. An electrocardiogram shows large S waves in leads V1 and V2, and large R waves in leads V4 through V6, with ST depressions in leads V5 and V6.
What is the most likely diagnosis?
A. Aortic stenosis.
B. Idiopathic hypertrophic subaortic stenosis (hypertrophic cardiomyopathy).
D. Pulmonary embolism.
E. Pulmonary hypertension.
The correct answer is A.
Kaplan Medical explains why
The presence of a systolic murmur over the aortic area that decreases as preload is decreased (i.e., with a Valsalva maneuver) is consistent with aortic stenosis. The fact that the murmur is late peaking suggests that the stenosis is significant. The prominent apical impulse and EKG suggests compensatory left ventricular hypertrophy (LVH).
The ST changes in leads V5 and 6 are often seen with left ventricular hypertrophy from systolic overload and are termed a "strain pattern." The presence of syncope with aortic stenosis significantly impacts the mortality associated with this disease. Data suggests that the average time to death from aortic stenosis with syncope is three years.
Why the other answers are wrong
Choice B: Idiopathic hypertrophic subaortic stenosis is unlikely since the murmur decreases with decreasing preload (the Valsalva maneuver). The outflow obstruction associated with idiopathic hypertrophic subaortic stenosis increases with decreasing preload and therefore, a decrease in preload is associated with an increase in the murmur intensity.
Choice C: Pericarditis is unlikely given the absence of chest pain, a precordial rub, or typical EKG findings (diffuse ST elevation, diffuse PR depression with PR elevation in aVL).
Choice D: Pulmonary embolism is unlikely given that the patient has no known history of risk factors for this process (e.g., hypercoagulability, poor activity), is not tachycardic or tachypneic, and has no EKG findings consistent with a pulmonary embolus (sinus tachycardia, S wave in lead I, Q wave and T wave inversion in lead III, or ST-T wave changes in leads V1 through V4).
Choice E: Pulmonary hypertension is unlikely given that there is no history of progressive dyspnea. The physical exam also does not show evidence of pulmonary hypertension (pulmonic tap, prominent pulmonic component of the second heart sound) or right heart strain (right ventricular heave, right sided S4, tricuspid regurgitation, etc.).
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
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