Question

Tyrosinosis is caused due to deficiency of which enzyme?

• A. Fumarylacetoacetate hydrolase
• B. p-hydroxy phenylpyruvate dehydrogenase
• C. Tyrosine transaminase
• D. Tyrosine ligase

Hint:

Think of the terminal enzyme of tyrosine catabolism whose loss causes liver failure (type I tyrosinemia).

Answer 1

The Correct Option is A

Step 1: Tyrosinosis is the same as type I tyrosinemia, an inherited error in the tyrosine catabolic pathway. The clinical clue is excess tyrosine in blood and tissues because the body cannot complete its breakdown.
Step 2: The block in type I tyrosinemia is at the final step of tyrosine catabolism, catalysed by fumarylacetoacetate hydrolase. Loss of this enzyme causes accumulation of toxic intermediates (fumarylacetoacetate and succinylacetone) that damage the liver and kidney.
Step 3: Management is a diet low in tyrosine and phenylalanine; untreated acute and chronic disease progresses to fatal liver failure. This confirms option (a).
Step 4: p-hydroxyphenylpyruvate dioxygenase deficiency causes type III tyrosinemia, and tyrosine transaminase deficiency causes type II (oculocutaneous) tyrosinemia, so (b) and (c) are wrong; "tyrosine ligase" (d) is not a recognised enzyme of this pathway.