USMLE PRACTICE QUESTION #114

TOPIC: Cardiac Anatomy

Dec 26, 2024

In USMLE practice question #114, we’ll test our knowledge of cardiovascular anatomy.

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A 57-year-old man with a history of coronary artery disease and hypertension presents with worsening dyspnea on exertion and fatigue over the past few weeks. Cardiac examination reveals a late diastolic murmur best heard at the apex. Echocardiography reveals left atrial enlargement and restricted motion of the anterior leaflet of the mitral valve. During cardiac surgery, the surgeon notes that a mass obstructing the inflow tract of the left ventricle prolapses into the left ventricle during diastole. This mass is most likely to originate from which of the following structures?

A) Crista terminalis
B) Left atrial appendage
C) Pulmonary veins
D) Fossa ovalis of the interatrial septum
E) Sinus venosus

Detailed Breakdown of Answers + Correct Answer Below ⏬

ANSWER + QUESTION BREAKDOWN

It’s important to adopt the correct MENTAL MODEL when answering USMLE questions; it saves time and increases accuracy. The mental model outlined below is a foundational component of our test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:

Step 1. Read the last line to get to the heart of the question: “This mass is most likely to originate from which of the following structures?”

Step 2: Is this a first-, second-, or third-order question?
Answer: 2n order. 1st: Diagnose the type of mass; 2nd: Determine the origin of the mass.

Step 3: Read the vignette carefully and ask yourself: “Based on the type of mass, it likely originates from _____________.”

Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.

GENERAL ANALYSIS

This vignette describes a 57-year-old man with a history of coronary artery disease and hypertension who presents with worsening dyspnea on exertion, fatigue, and a productive cough. The patient has a late diastolic murmur heard best at the apex, and echocardiography shows left atrial enlargement and restricted motion of the anterior leaflet of the mitral valve. During cardiac surgery, a mass is noted to be obstructing the inflow tract of the left ventricle and prolapsing into the left ventricle during diastole. This description is highly suggestive of a left atrial myxoma, which is the most common primary cardiac tumor.

ANSWER CHOICES:

CHOICE A: Crista terminalis

Explanation: The crista terminalis is a muscular ridge located in the right atrium, separating the smooth part of the right atrium from the trabeculated part.

Why Incorrect: Myxomas typically arise in the left atrium near the interatrial septum, not from the crista terminalis in the right atrium. Therefore, this is not a likely origin for this mass.

CHOICE B: Left atrial appendage

Explanation: The left atrial appendage is an outpouching of the left atrium that can be a site for thrombus formation in conditions like atrial fibrillation.

Why Incorrect: While masses such as thrombi can form in the left atrial appendage, myxomas typically arise from the interatrial septum, particularly near the fossa ovalis. This makes it less likely that this mass originated from the left atrial appendage.

CHOICE C: Pulmonary veins

Explanation: The pulmonary veins drain oxygenated blood from the lungs into the left atrium.

Why Incorrect: While tumors can occasionally involve pulmonary veins, myxomas are most commonly found attached to the interatrial septum rather than originating from pulmonary veins.

CHOICE D: Fossa ovalis of the interatrial septum

Explanation: The fossa ovalis is a depression in the interatrial septum that marks where the foramen ovale was located in fetal circulation. It is a common site for myxoma attachment.

Why Correct: The majority of left atrial myxomas originate from the fossa ovalis region of the interatrial septum. These tumors can prolapse into the mitral valve orifice during diastole, causing obstruction and mimicking mitral valve disease.

CHOICE E: Sinus venosus

Explanation: The sinus venosus is an embryological structure that forms part of the right atrium and contributes to venous return to the heart.

Why Incorrect: Myxomas do not typically arise from this structure. Instead, they commonly originate from the interatrial septum near the fossa ovalis.

FINAL VERDICT…

The most likely diagnosis for this patient is a left atrial myxoma, which typically arises from the fossa ovalis region of the interatrial septum. Myxomas are benign tumors but can cause significant symptoms by obstructing blood flow through the heart. In this case, during diastole, when blood flows from the left atrium to the left ventricle, this mass prolapses into the mitral orifice, mimicking mitral valve disease and causing symptoms such as dyspnea and fatigue.

KEY CONCEPTS:

The patient's symptoms (dyspnea, fatigue, and positional symptoms) are consistent with an obstructive mass in the heart.
The left atrial myxoma typically arises from the interatrial septum, specifically near the fossa ovalis, and can prolapse into the mitral valve orifice during diastole, mimicking mitral valve disease.
The presence of a mass obstructing the inflow tract of the left ventricle that prolapses into the ventricle during diastole is characteristic of a myxoma.

CORRECT ANSWER: D) Fossa ovalis of the interatrial septum