Question

A 72-year-old man with a long-standing history of tobacco use disorder, emphysema, and ischemic cardiomyopathy presents to the emergency department complaining of acutely worsening shortness of breath for the past 24 hours. He appears visibly distressed and is leaning forward in the tripod position. Vital signs show a temperature of 37.2°C (99.0°F), blood pressure of 145/88 mm Hg, pulse of 105/min, and respiratory rate of 28/min. Pulse oximetry reveals an oxygen saturation of 88% on room air. Physical examination demonstrates diffuse expiratory wheezing bilaterally, but auscultation of the heart sounds is difficult due to the noisy lung fields. The patient has mild bilateral pitting edema at the ankles, which he states is chronic. Given the overlapping clinical features of his comorbidities, which of the following laboratory tests is most useful to rapidly determine whether his acute presentation is primarily driven by decompensated heart failure rather than a pulmonary exacerbation?