Essential atrophy of the choroid is due to inborn error of metabolism of which amino acid?
**Core Concept**
Essential atrophy of the choroid, also known as gyrate atrophy, is a rare inherited eye disorder characterized by progressive vision loss due to degeneration of the choroid and retina. This condition is caused by a genetic mutation affecting the metabolism of a specific amino acid.
**Why the Correct Answer is Right**
The correct answer is D. Ornithine, as gyrate atrophy is caused by a deficiency in the enzyme ornithine aminotransferase (OAT), which is responsible for the breakdown of ornithine. Accumulation of ornithine leads to oxidative damage and atrophy of the choroid and retina, resulting in progressive vision loss. Ornithine is an amino acid involved in various metabolic pathways, including the urea cycle, where it is converted to citrulline by ornithine transcarbamylase (OTC) or to alpha-ketoglutarate and glutamate by ornithine aminotransferase (OAT).
**Why Each Wrong Option is Incorrect**
**Option A:** Cystine is an amino acid involved in the metabolism of cysteine, and its deficiency is associated with cystinuria, a different inherited disorder characterized by the formation of cystine stones in the kidneys and urinary tract.
**Option B:** Cysteine is an amino acid that is converted to cystine through the enzyme cysteine dioxygenase, and its metabolism is not directly related to gyrate atrophy.
**Option C:** Arginine is an amino acid involved in the urea cycle, where it is converted to urea and ornithine by the enzyme arginase. While arginine metabolism is related to ornithine, its deficiency is not associated with gyrate atrophy.
**Clinical Pearl / High-Yield Fact**
Gyrate atrophy is an X-linked recessive disorder, meaning it primarily affects males, who have only one X chromosome. Female carriers may exhibit mild symptoms due to X-chromosome inactivation.
**✓ Correct Answer: D. Ornithine**