Question

Fish odor syndrome is caused by deficiency of which enzyme?

• A. Fumarylacetoacetate hydrolase
• B. Methane monooxygenase
• C. Flavin monooxygenase 3 (FMO3)
• D. D-amino acid oxidase

Hint:

Which liver monooxygenase converts smelly trimethylamine into odourless TMAO?

Answer 1

The Correct Option is C

Step 1: Fish odor syndrome (trimethylaminuria) is defined by a fishy smell in urine, sweat, and breath caused by accumulation of trimethylamine (TMA), a volatile tertiary amine that smells like rotting fish.
Step 2: Normally TMA is oxidised in the liver by flavin monooxygenase 3 (FMO3) to odourless trimethylamine-N-oxide (TMAO). A deficiency of FMO3 leaves TMA unoxidised, so it is excreted unchanged and produces the odour.
Step 3: Primary trimethylaminuria is autosomal recessive due to FMO3 mutations; diagnosis is by clinical odor plus urinary TMA/TMAO ratio. This makes (c) correct.
Step 4: Fumarylacetoacetate hydrolase belongs to tyrosine catabolism, methane monooxygenase is a bacterial enzyme, and D-amino acid oxidase has no role in TMA handling, so (a), (b), and (d) are wrong.