This case presentation discusses an extremely rare diagnosis presenting with common symptoms, attributed to influenza, which were not investigated further when clear cardiac symptoms developed. The patient initially presented with symptoms consistent with influenza, but when orthopnea and dyspnea on exertion developed, these cardiac symptoms continued to be attributed to a postviral syndrome and were not further evaluated. Premature closure bias contributed to a delay in diagnosing a rare cardiac condition. The diagnostic momentum, or the continuing of a diagnostic label, occurred across multiple providers and settings. This case demonstrates the risk of premature closure and diagnostic momentum and reminds clinicians to reframe the differential diagnosis as more information on history or physical exam becomes available.