USMLE-Rx Step 1 Qmax Challenge #1690

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USMLE-Rx Step 1 Qmax Challenge #1690A 38-year-old diabetic woman gives birth to a term male infant. Immediately after birth, the infant is cyanotic and tachypneic. A murmur is not auscultated during the newborn screening examination. His hypoxemia quickly worsens over minutes, and a chest radiograph is obtained, which resembles the results shown here for an older child. He undergoes cardiac catheterization, in which a balloon is guided to perforate the atrial septum. He is also given an infusion of prostaglandin E1. An ECG after the procedure demonstrates right-axis deviation and right ventricular hypertrophy. The infant’s hypoxia stabilizes, and he later undergoes definitive, corrective surgery.

Which of the following is the underlying condition responsible for this infant’s hypoxemia?

A. Concomitant ventricular septal defect
B. Pulmonary atresia
C. Tetralogy of Fallot
D. Total anomalous pulmonary venous return
E. Transposition of the great vessels
F. Tricuspid atresia


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7 thoughts on “USMLE-Rx Step 1 Qmax Challenge #1690”

  1. Melanie A Lee MD

    I believe it’s A- concomitant ventricular septal defect. With a Diabetic Mother, there would be hypertrophic enlargement of the septum in the fetus. It’s triggered by hyperglycemia during the third trimester.

  2. The correct answer is E. The neonate has transposition of the great vessels, in which the aorta arises from the right ventricle and the pulmonary artery from the left ventricle. Risk factors leading to this common congenital heart defect include maternal diabetes. Without a shunt, transposition is incompatible with life. If the infant is born without the shunt, an artificial shunt can be created by balloon atrial septostomy, and prostaglandin E1 can be given to salvage whatever patent ductus arteriosus might remain. Once stabilized, the infant then can undergo corrective surgery.

    Several clues in the chest radiograph suggest transposition of the great vessels (TGV). First, the heart is egg- or dome-shaped. Second, the superior mediastinum is narrowed. This combination of findings yields an “egg on a string” appearance, which is characteristic of TGV. Finally, the pulmonary vascular markings are increased. The patient in this radiograph also has a right aortic arch, which can be seen with TGV and truncus arteriosus.

    For Step 1 it is important to know the “5 T’s” of neonatal cyanosis: Tetralogy of Fallot, Transposition of the great vessels, Truncus arteriosus, Tricuspid atresia, and Total anomalous pulmonary venous return (TAPVR). Key to differentiating transposition of the great vessels (TGV) from tetralogy of Fallot (TOF) is that TGV presents immediately after birth, whereas TOF presents later.

    A is not correct. A concomitant ventricular septal defect (VSD), although sometimes present in patients with transposition of the great vessels, is not the underlying pathophysiology of this infant’s cyanosis. Additionally, if this patient had a VSD, there would have been little reason to create an additional shunt in the atria.

    B is not correct. Newborns with pulmonary atresia would present with early cyanosis as the patent ductus arteriosus closes, hypotension, tachypnea, a single S2, and a holosystolic murmur resulting from tricuspid regurgitation from increased right end-systolic ventricular pressure. X-ray of the chest may demonstrate a large right ventricle without prominent pulmonary vasculature.

    C is not correct. Tetralogy of Fallot (TOF) presents in a varied manner, dependent upon the magnitude of right ventricular outflow tract obstruction. In severe cases, in which pulmonary circulation is significantly decreased, newborns present immediately with profound cyanosis.

    D is not correct. In total anomalous pulmonary venous return, the pulmonary veins do not drain into the left atrium. The most common site for the pulmonary veins to drain is the superior vena cava. On examination, newborns will have a hyperactive right ventricular impulse, quadruple rhythm, and a fixed and split S2. There also will be a prominent systolic ejection murmur at the left upper sternal border and a mid-diastolic rumble at the left lower sternal border. The ejection murmur is due to increase stroke volume across pulmonary valve; the mid-diastolic rumble at the left lower sternal border is due to increased flow across the tricuspid.

    F is not correct. For tricuspid atresia, typically an atrial septal defect or a ventricular septal defect is present. Most newborns (approximately 75%) will present with cyanosis in the first week of life.

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