**Core Concept**
Carcinoid tumors, especially those arising from the lungs or appendix, secrete excessive amounts of serotonin and its metabolite 5-hydroxyindoleacetic acid (5HIAA). The diagnosis is primarily based on detecting elevated serum or urinary 5HIAA levels, which reflect serotonin metabolism.

**Why the Correct Answer is Right**
5HIAA is the primary urinary metabolite of serotonin, and carcinoid tumors often produce large amounts of serotonin. Elevated 5HIAA in urine (typically >100 µg/day) is a key diagnostic marker. The test is most reliable when performed over 24 hours and is particularly useful in patients with flushing, diarrhea, or wheezing suggestive of carcinoid syndrome. It is a non-invasive, specific screening tool for neuroendocrine tumors.

**Why Each Wrong Option is Incorrect**
Option B: DHEA (dehydroepiandrosterone) is a precursor of sex hormones and is elevated in adrenal disorders like congenital adrenal hyperplasia or polycystic ovary syndrome, not in carcinoid tumors.
Option C: DHEA is identical to option B and is irrelevant to carcinoid tumor diagnosis.
Option D: Metanephrines are biomarkers for pheochromocytoma, a catecholamine-secreting tumor of the adrenal medulla, not carcinoid tumors.

**Clinical Pearl / High-Yield Fact**
In patients with suspected carcinoid syndrome, a 24-hour urine 5HIAA test is the first-line screening tool. A level above 100 µg/day is diagnostic, and further imaging (like CT or octreotide scan) is needed for localization.

✓ Correct Answer: A. 5HIAA