Congenital Heart Disease USMLE Step 1 Practice Question
A 5-year-old boy with tetralogy of Fallot presents to the emergency department with acute dyspnea and presyncope during soccer practice. His parents note he frequently assumes a squatting position when fatigued during play. On examination, the child is cyanotic with oxygen saturation of 78% on room air. Cardiac auscultation reveals a single loud S2 and a systolic ejection murmur. Chest X-ray demonstrates a boot-shaped cardiac silhouette with decreased pulmonary vascular markings. Which of the following physiologic mechanisms explains the symptomatic benefit of the squatting posture in this patient?
Answer choices
- ADecreased pulmonary vascular resistance that facilitates left-to-right shunting across the VSD
- BIncreased left ventricular preload that augments systemic cardiac output
- CIncreased systemic vascular resistance that reduces the right-to-left shunt fractionCorrect answer
- DDecreased intrathoracic pressure that improves coronary perfusion pressure
- ELower extremity venous compression that increases pulmonary blood flow
- FReduced metabolic oxygen consumption that decreases the severity of cyanosis
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