Valvular Heart Disease USMLE Step 1 Practice Question
A 12-year-old boy presents with a heart murmur detected on routine examination. Vital signs show BP 110/72 mmHg, HR 88 bpm, RR 18/min, temperature 37°C, and SpO2 98% on room air. Physical examination reveals a fixed split S2 that does not vary with respiration. Chest X-ray shows mild cardiomegaly. Transthoracic echocardiography confirms a secundum atrial septal defect with a 15-mm defect size and normal left ventricular function. He denies dyspnea on exertion. Which mechanism best explains the fixed splitting of S2?
Answer choices
- ARight ventricular stroke volume remains persistently increased throughout the respiratory cycleCorrect answer
- BInspiration equally increases both left and right ventricular filling
- CPulmonic valve closure is fixed early because pulmonary artery pressure is low
- DThe aortic valve closes late due to left ventricular outflow obstruction
- EThe murmur is generated by turbulence across the septal defect itself
- FRight bundle branch block causing delayed RV activation and fixed S2 splitting
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