Nonbacterial Thrombotic Endocarditis Unmasking Concomitant Monoclonal Gammopathy of Undetermined Significance (MGUS) by Manifesting as Stroke.

Nonbacterial thrombotic endocarditis (NBTE) is a rare condition characterized by sterile platelet-fibrin vegetations on cardiac valves in the absence of systemic infection. The pathogenesis of marantic endocarditis is driven by endothelial dysfunction and a systemic hypercoagulable state. In contrast to infective endocarditis, vegetations in NBTE lack significant inflammatory infiltrates and do not yield positive blood cultures. NBTE typically comes to clinical attention via systemic embolic events, with cerebrovascular accidents serving as a clinical hallmark and constituting over 50% of cases. While NBTE is commonly associated with mucin-producing adenocarcinomas of the lung, pancreas, and gastrointestinal tract, its occurrence secondary to hematological malignancies or precursor plasma cell dyscrasias like monoclonal gammopathy of undetermined significance (MGUS) is exceedingly rare, particularly in young individuals. We report the case of a previously healthy 35-year-old woman who presented with acute-onset blurred vision. Neuroimaging via magnetic resonance imaging (MRI) revealed an acute left occipital infarct along with multiple chronic infarcts, raising a strong suspicion of a recurrent embolic process. A transesophageal echocardiogram (TEE) demonstrated two vegetations on the aortic valve with moderate transvalvular regurgitation; in the context of persistent negative blood cultures, these findings supported the diagnosis of NBTE. The patient was managed with systemic anticoagulation. A comprehensive hypercoagulable and autoimmune workup revealed an elevated lambda free light chain level with a decreased kappa/lambda ratio. Subsequent bone marrow biopsy and cytogenetic analysis established a diagnosis of MGUS featuring high-risk genomic aberrations, specifically an immunoglobulin heavy chain/musculoaponeurotic fibrosarcoma (IGH/MAF) rearrangement and the loss of chromosome 13 in a polyploid (3n, 4n) background, findings consistent with plasma cell neoplasia. The prevalence of MGUS in individuals under the age of 40 is exceptionally low, estimated at less than 0.3%. This case underscores NBTE as a critical finding that can unmask underlying, atypical plasma cell neoplasms. It highlights the necessity of an exhaustive diagnostic evaluation for occult hematological disorders and high-risk cytogenetic features in young patients presenting with multi-territory embolic strokes.