Question

5-hydroxyindoleacetic acid (HIAA) is present in urine in which condition?

• A. Alkaptonuria
• B. Albinism
• C. Carcinoid syndrome
• D. Phenylketonuria

Hint:

5-HIAA is a serotonin metabolite, so think of a serotonin-secreting tumour.

Answer 1

The Correct Option is C

Step 1: 5-hydroxyindoleacetic acid (5-HIAA) is the main urinary breakdown product of serotonin (5-HT).

Step 2: Carcinoid tumours secrete large amounts of serotonin and other vasoactive substances, producing carcinoid syndrome with flushing, abdominal cramps and diarrhoea. The excess serotonin is metabolised to 5-HIAA, which rises in urine. So the answer is carcinoid syndrome.

Step 3: Measuring 24-hour urinary 5-HIAA is the standard biochemical test to diagnose carcinoid syndrome.

Step 4: The distractors involve other pathways: alkaptonuria raises homogentisic acid, albinism is a defect of melanin synthesis (tyrosinase), and phenylketonuria raises phenylketones, none of which produces 5-HIAA.