If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 2 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 4,000-gram male neonate develops severe cyanosis that begins within minutes of birth. Blood drawn one hour after birth shows metabolic acidosis with respiratory acidosis. A chest radiograph shows a narrow mediastinum, narrow heart base, and absence of the pulmonary artery. An electrocardiogram is normal. An echocardiogram is ordered and the report is pending.
Which of the following is the most likely diagnosis?
A. Aortic valve stenosis.
B. Complete atrioventricular canal defect.
C. Tetralogy of Fallot.
D. Transposition of the great arteries.
E. Underdeveloped (hypoplastic) left ventricle syndrome.
The correct answer is D.
Kaplan Medical explains why
This is transposition of the great arteries, in which the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle. Approximately 5% of congenital cardiac anomalies have transposition of the great arteries. Affected infants present within minutes of birth with severe cyanosis and metabolic acidosis secondary to inability to oxygenate tissues. The only exchange of blood between pulmonic and circulatory systems is typically occurring through a patent ductus arteriosus. The chest radiograph changes include a narrow mediastinum, narrow heart base, and absence of the pulmonary artery; these are caused by superposition of the great vessels (rather than the normal side-to-side position). Surgical repair is usually performed within 7 to 10 days of life.
Why the other answers are wrong
Choice A: Aortic valve stenosis produces a loud ejection murmur with a prominent systolic click heard best at the upper right sternal border. This does not cause cyanosis or metabolic acidosis.
Choice B: Complete atrioventricular canal defect can also cause cyanosis at birth, but will show marked ECG changes, sometimes including absent Q waves.
Choice C: Tetralogy of Fallot may present at birth, with ECG changes showing right ventricular hypertrophy and right axis deviation. Chest radiographs usually show a small heart with a concave main pulmonary artery.
Choice E: Underdeveloped left ventricle syndrome (also known as hypoplastic left heart syndrome) causes an abrupt onset of severe heart failure with loss of peripheral pulses at 2-3 days of life. These patients have an intact ventricular septum but underdeveloped left ventricle and mitral and aortic valves. Coarctation of the aorta commonly accompanies this condition. Newborns usually appear normal after birth and for several days because they are dependent on a patent ductus arteriosus. Once the ductus arteriosus closes, they develop shock. Examination reveals diminished pulses and increased right ventricular impulse.
Tips to remember
In transposition of great vessels, because the aorta is coming out of the right ventricle it will completely cover the pulmonary trunk. This will be seen as absence of the pulmonary artery with a narrow mediastinum on chest radiograph ("egg on a string" appearance).
With this transposition, the pulmonary artery comes out of the left ventricle. Because of these changes, deoxygenated systemic venous blood travels from the right ventricle to the aorta without passing through the lungs, and pulmonary venous blood travels from the left ventricle to the lungs via the pulmonary artery. The systemic and pulmonary circulations parallel each other rather than occur sequentially. This is not compatible with life unless the two chambers connect via VSD, ASD, or patent ductus arteriosus. Prostaglandin E1 is used to maintain patency of the ductus arteriosus. An arterial switch operation is performed in the newborn period most commonly.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
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