A 45-year-old woman presents with a 3-month history of progressive dyspnea, orthopnea, and intermittent palpitations. On auscultation, a low-pitched early diastolic sound is heard after S2, followed by a mid-diastolic rumble. Transthoracic echocardiography reveals a pedunculated, gelatinous mass arising from the interatrial septum at the fossa ovalis, prolapsing through the mitral valve during diastole. The structure marked **B** in the diagram is most consistent with which diagnosis, and what is the primary mechanism of her obstructive symptoms?

Question

Accepted Answer

C. Cardiac myxoma; the pedunculated mass prolapses through the mitral valve during diastole, mimicking mitral stenosis and producing the characteristic 'tumor plop' sound. ## Why option 1 is correct

The structure marked **B** is a cardiac myxoma located in the left atrium at the fossa ovalis, the most common site (>75% of cases). The clinical presentation is pathognomonic: the pedunculated tumor prolapses through the mitral valve during diastole, mechanically obstructing left ventricular filling and producing the classic triad of obstructive symptoms (dyspnea, orthopnea, pulmonary edema), embolic phenomena, and constitutional symptoms. The low-pitched early diastolic sound ("tumor plop") is the hallmark auscultatory finding, occurring as the mass moves through the mitral valve. Cardiac myxoma is the most common primary benign cardiac tumor in adults (~50% of all primary cardiac tumors), with peak incidence 30–60 years and female predominance. The diagnosis is confirmed by echocardiography showing the characteristic pedunculated gelatinous mass with a narrow stalk attached to the interatrial septum. [Braunwald Heart Disease, 11th ed; WHO Cardiac Tumors Classification]

## Why each distractor is wrong

- **Option 2 (Papillary fibroelastoma)**: This is a small (<2 cm), mobile, valve-attached tumor that produces systolic murmurs, not the early diastolic "tumor plop" sound. It arises from valve leaflets, not the interatrial septum, and does not cause the obstructive pattern described.
- **Option 3 (Cardiac rhabdomyoma)**: This is a hamartomatous lesion typically arising from the ventricular wall (not the atrial septum) and is the most common cardiac tumor in *children*, not adults. It does not produce the characteristic diastolic plop or mimic mitral stenosis.
- **Option 4 (Metastatic carcinoma)**: While metastases can occur in the heart, they do not typically present as a solitary pedunculated mass at the fossa ovalis with a narrow stalk. Metastases are usually multiple and do not produce the classic "tumor plop" sound.

**High-Yield:** Cardiac myxoma at the fossa ovalis with a pedunculated stalk prolapsing through the mitral valve = "tumor plop" + mitral stenosis mimicry + systemic embolization risk in a middle-aged woman.

[cite: Braunwald Heart Disease, 11th ed; WHO Cardiac Tumors Classification]

Answer

A. Papillary fibroelastoma; small mobile mass attached to the valve leaflet causing intermittent obstruction and a systolic ejection murmur

Answer

B. Cardiac rhabdomyoma; a hamartomatous lesion arising from the ventricular wall that obstructs left ventricular outflow

Answer

D. Metastatic carcinoma; a friable tumor with surface thrombus that embolizes to the pulmonary circulation causing dyspnea

Explanation

Why option 1 is correct

Why each distractor is wrong

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