Question:
Ochronosis is due to the accumulation of which of the following?
Ochronosis is due to the accumulation of which of the following?
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Same metabolite that blackens urine on standing in alkaptonuria.
Updated On: Jun 23, 2026
- Homogentisic acid
- Homocysteine
- Xanthurenate
- Glyoxylate
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The Correct Option is A
Solution and Explanation
Step 1: Ochronosis is the bluish-black pigmentation of connective tissue seen in alkaptonuria.
Step 2: Alkaptonuria is an autosomal recessive defect of homogentisate 1,2-dioxygenase. Homogentisic acid accumulates and is excreted in urine (which blackens on standing). Over years it is oxidized by polyphenol oxidase and polymerizes to dark alkapton pigment that deposits in cartilage and connective tissue. That deposition is ochronosis, so option (a), homogentisic acid, is correct.
Step 3: The others are wrong: homocysteine accumulates in homocystinuria, xanthurenate appears with vitamin B6 deficiency in tryptophan metabolism, and glyoxylate links to primary hyperoxaluria. None cause ochronosis.
Note: The original recall paper listed two identical options; the second has been corrected to homocysteine to keep four distinct choices, which does not change the answer.
Clinical: Ochronosis appears by the third or fourth decade with pigment in intervertebral discs, nasal cartilage and ear pinna, plus arthritis. Ref: DM Vasudevan, Textbook of Biochemistry, 6e, p 208.
Step 2: Alkaptonuria is an autosomal recessive defect of homogentisate 1,2-dioxygenase. Homogentisic acid accumulates and is excreted in urine (which blackens on standing). Over years it is oxidized by polyphenol oxidase and polymerizes to dark alkapton pigment that deposits in cartilage and connective tissue. That deposition is ochronosis, so option (a), homogentisic acid, is correct.
Step 3: The others are wrong: homocysteine accumulates in homocystinuria, xanthurenate appears with vitamin B6 deficiency in tryptophan metabolism, and glyoxylate links to primary hyperoxaluria. None cause ochronosis.
Note: The original recall paper listed two identical options; the second has been corrected to homocysteine to keep four distinct choices, which does not change the answer.
Clinical: Ochronosis appears by the third or fourth decade with pigment in intervertebral discs, nasal cartilage and ear pinna, plus arthritis. Ref: DM Vasudevan, Textbook of Biochemistry, 6e, p 208.
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