This patient most likely has a myxoma that is causing her symptoms and clinical findings. Left atrial myxomas are the most common benign tumors of the heart. These lesions can cause constitutional symptoms, such as fatigue, dyspnea, fever, and weight loss, related to tumor cytokine production; systemic embolization from either tumor fragments or associated thrombi may cause neurologic symptoms or other systemic sequelae. Left atrial myxomas most commonly appear as a mass arising from the atrial septum; the mass can involve the mitral valve intermittently to cause a “tumor plop” that may clinically mimic mitral stenosis. Myxomas may occur as part of the Carney complex, which is an autosomal dominant disorder associated with pigmentation abnormalities (such as blue nevi), schwannomas, and endocrine tumors.

Metastatic adenocarcinoma with cardiac involvement can manifest from direct invasion or hematogenous spread, with symptoms and signs dependent on the site of involvement. Metastatic adenocarcinoma with cardiac involvement, although common in patients with this tumor type (15% of patient at autopsy), would be less likely in this patient given the isolated anatomic location and recent negative malignancy screening results.

Angiosarcomas are malignant tumors that can occur in the atria but are less common than myxomas and typically infiltrate the myocardium, which is normal in this patient.

Lipomas typically are located in the subendocardium, not the atrium, and rarely cause symptoms.

Papillary fibroelastomas, like myxomas, can be mobile with a pedunculated stalk. However, they most commonly arise on left-sided cardiac valves and the left ventricular outflow tract.