USMLE (United States Medical Licensing Examination) Step 1 Practice Exam

In this context, an elevated alveolar-arterial (A-a) gradient indicates that there is impaired gas exchange in the lungs, and it can help differentiate between different types of hypoxemia. A shunt, whether intracardiac or intrapulmonary, allows blood to bypass the normal gas exchange process, leading to an altered A-a gradient.

An intracardiac shunt, such as a ventricular septal defect (VSD), is characterized by a direct passage of blood from the right side of the heart to the left side without being oxygenated in the lungs. This bypass results in poorly oxygenated blood entering systemic circulation, leading to desaturation and, consequently, an elevated A-a gradient because the alveoli are not delivering sufficient oxygen to the mixed venous blood returning to the left atrium.

While intrapulmonary shunts (like those caused by pulmonary arteriovenous malformations, pulmonary edema associated with congestive heart failure, or pneumonia) also cause elevated A-a gradients, the question specifically refers to an elevated A-a gradient associated with a shunt condition - which highlights the fact that VSD directly allows for bypassing oxygenation across cardiac structures.

Conditions such as atelectasis and pulmonary fibrosis primarily lead