In which disorder will urine turn black if the sample's pH becomes alkaline upon standing?

In which disorder will urine turn black if the sample's pH becomes alkaline upon standing?

• A. Malignant melanoma
• B. Hemolytic anemia
• C. Alkaptonuria
• D. Hepatitis

Answer: (C)

In alkaptonuria, the body lacks the enzyme that breaks down tyrosine, so homogentisic acid builds up and is excreted in the urine. When the urine sits and the environment becomes alkaline, homogentisic acid oxidizes and polymerizes to a dark pigment, causing the urine to turn black upon standing. This characteristic darkening is a classic clue to the disorder. The other conditions do not produce this specific standing-alkaline darkening: hemolytic anemia can give dark urine from free hemoglobin but not a pigment formed by standing; hepatitis darkens urine due to bilirubin, not due to standing-induced oxidation; malignant melanoma can color urine but not through this standing-alakaline oxidation mechanism.

In alkaptonuria, the body lacks the enzyme that breaks down tyrosine, so homogentisic acid builds up and is excreted in the urine. When the urine sits and the environment becomes alkaline, homogentisic acid oxidizes and polymerizes to a dark pigment, causing the urine to turn black upon standing. This characteristic darkening is a classic clue to the disorder. The other conditions do not produce this specific standing-alkaline darkening: hemolytic anemia can give dark urine from free hemoglobin but not a pigment formed by standing; hepatitis darkens urine due to bilirubin, not due to standing-induced oxidation; malignant melanoma can color urine but not through this standing-alakaline oxidation mechanism.