Which of the following is NOT a non-cardiac cause of elevated BNP levels?

The correct choice indicates that heart murmur is not a non-cardiac cause of elevated BNP levels. B-type Natriuretic Peptide (BNP) is primarily released by the ventricles of the heart in response to volume expansion and pressure overload, typically due to heart failure or cardiac-related conditions. Therefore, heart murmurs, which often indicate underlying cardiac issues such as valvular heart disease or structural heart problems, are directly related to the heart's performance and can contribute to elevated BNP levels as a physiological response.

In contrast, non-cardiac causes of elevated BNP, such as renal failure, severe pneumonia, and obstructive sleep apnea, are not primarily linked to heart workload but can still lead to increased BNP levels due to the body's stress response or fluid overload. For instance, renal failure can cause fluid retention leading to increased pressures that may elevate BNP, while conditions like severe pneumonia can stress the body and potentially result in secondary effects on BNP levels. Thus, the distinction arises because heart murmurs reflect cardiovascular pathology, which makes their association with elevated BNP levels direct, rather than non-cardiac.