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: Start from the parent molecule. $5\text{-HIAA}$ is the principal urinary metabolite of serotonin, so a high urinary $5\text{-HIAA}$ signals serotonin overproduction.

Step 2: Scan the options for a serotonin excess. Carcinoid tumours pour out serotonin along with bradykinin and histamine, causing flushing, cramps and diarrhoea, and the serotonin is converted to $5\text{-HIAA}$ in urine. Hence carcinoid syndrome is correct.

Step 3: Clinically, a 24-hour urinary $5\text{-HIAA}$ measurement is the key screening test for this syndrome.

Step 4: Reject the distractors by their own markers: homogentisic acid in alkaptonuria, absent melanin in albinism, and phenyl ketoacids in phenylketonuria, none yielding $5\text{-HIAA}$.

\[\boxed{\text{Carcinoid syndrome}}\]