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

What is a characteristic of a left-to-right shunt?

1. May lead to a right-to-left shunt

2. Bypasses the systemic circulation

3. Associated with restrictive lung disease

4. Is not influenced by pulmonary hypertension

The correct answer is: May lead to a right-to-left shunt

A left-to-right shunt occurs when blood flows from the left side of the heart to the right side, typically due to a congenital heart defect such as a ventricular septal defect or an atrial septal defect. This can lead to increased blood flow to the lungs, which over time can result in pulmonary hypertension. If the pressure in the right side of the heart becomes greater than that of the left side, it can culminate in a reversal of the shunt direction, leading to a right-to-left shunt. This transition is characterized by the mixing of deoxygenated and oxygenated blood, which can significantly impact oxygen delivery to the body. The other choices refer to characteristics that do not typically apply to left-to-right shunts. For instance, a left-to-right shunt does not bypass systemic circulation; rather, it diverts blood back to the lungs before it can return to the systemic circulation. Associating a left-to-right shunt with restrictive lung disease is not accurate as these conditions pertain to the lungs' compliance and are not a direct result of the hemodynamics involved in a left-to-right shunt. Additionally, while pulmonary hypertension can influence the dynamics of a shunt, it is particularly relevant to understanding the potential