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A 38-year-old male has severe arthritis of his spine, knees, and hips. He is still able to ambulate but must walk with a cane. He presents to primary care clinic for a routine physical exam. A complete blood count is normal, his electrolytes are within normal limits, and his cholesterol levels are normal. The patient leaves a urine sample that turns black on standing. Analysis of the urine will most likely show which of the following?
A. Elevated uric acid levels
B. Positive leukocyte esterase
C. Hematuria
D. Elevated homogentisic acid levels
E. Proteinuria
F. Elevated phenylalanine levels
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D. Elevated homogentisic acid levels
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Choice (D) is the correct answer. Alkaptonuria, also known as black urine disease, is a disease caused by a deficiency of the enzyme homogentisic oxidase. Homogentisic acid accumulates in the disease and is a toxic byproduct of tyrosine catabolism. It is excreted in the urine, and patients with alkaptonuria have urine that turns black on standing. Homogentisic acid is harmful to cartilage and bone, and patients develop debilitating arthritis known as ochronosis arthritis. Ochronosis refers to the dark pigmentation of cartilage and fibrous tissue that is seen in alkaptonuria as a result of homogentisic acid accumulation. Newborns are generally asymptomatic, except they have diapers that turn black on standing. Arthritis develops later in life, usually around the third decade. Elevated uric acid levels (Choice A) in the urine is associated with gout, though not every patient with gout has hyperuricemia. Urine positive for leukocyte esterase (Choice B) is associated with urinary tract infections. Hematuria (Choice C) is associated with many conditions, some of which include prostate cancer, bladder cancer, nephritic disease, and trauma. Proteinuria (Choice E) is also seen in a number of conditions including nephrotic syndromes and diabetes. (Choice F) is incorrect.
