The nurse is caring for a client diagnosed with acute heart failure on the telemetry unit. Which laboratory test does the nurse use to analyze the severity of this condition?

A. B-type natriuretic peptide (BNP) levels increase in response to ventricular volume expansion and pressure overload, making it a key marker for diagnosing and assessing the severity of heart failure. B. Hemoglobin assesses oxygen-carrying capacity but does not indicate heart failure severity. C. Potassium is important for cardiac function but not a direct measure of heart failure severity. D. Sodium levels can be affected in heart failure but are not the primary indicator of severity.