Final answer:
The nurse assesses B-type natriuretic peptide (BNP) to determine the severity of heart failure. BNP levels correlate with the severity of the condition, unlike BUN, electrolytes, or CBC tests.
Step-by-step explanation:
To determine the severity of a patient's heart failure secondary to dilated cardiomyopathy, the key diagnostic test that a nurse assesses is B-type natriuretic peptide (BNP). This is because BNP is a hormone released by the ventricles in response to increased wall tension that occurs with heart failure. High levels of BNP are indicative of heart failure and correlate with its severity. In contrast, other tests such as Blood urea nitrogen (BUN), Serum electrolytes, and Complete blood count (CBC) can provide ancillary information but they do not directly assess heart failure severity.