Alkaptonuria md pathology exam question give robbins based answer
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Robbins pathology alkaptonuria homogentisic acid oxidase ochronosis
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Alkaptonuria
Definition:
Alkaptonuria is a rare autosomal recessive inborn error of tyrosine metabolism caused by deficiency of homogentisate 1,2-dioxygenase (homogentisic acid oxidase), encoded by the HGD gene.
Alkaptonuria is a rare autosomal recessive inborn error of tyrosine metabolism caused by deficiency of homogentisate 1,2-dioxygenase (homogentisic acid oxidase), encoded by the HGD gene.
Pathogenesis
Normal pathway:
Phenylalanine/Tyrosine → Homogentisic acid → Maleylacetoacetic acid
In alkaptonuria, homogentisic acid cannot be metabolized because of homogentisate oxidase deficiency. Therefore, homogentisic acid accumulates and:
- Is excreted in large amounts in urine.
- Oxidizes on exposure to air, causing the urine to become brown-black or black on standing.
- Oxidizes and polymerizes in tissues, forming a dark pigment that binds particularly to collagen-rich connective tissues.
Ochronosis
Deposition of the dark homogentisic acid-derived pigment in cartilage, connective tissue, and skin is called ochronosis.
Robbins describes homogentisic acid as a black endogenous pigment in alkaptonuria, deposited in skin, connective tissue, and cartilage, producing ochronosis. Robbins, Cotran & Kumar Pathologic Basis of Disease, Endogenous Pigments.
Morphology and clinical features
- Darkening of urine on standing, often noticed early in life.
- Bluish-black pigmentation of cartilage and connective tissue:
- Pinnae of ears
- Sclera
- Nose
- Skin
- Ochronotic arthropathy develops later, usually in adulthood:
- Pigmented, brittle cartilage
- Degenerative arthritis, especially of the spine, hips, knees, and shoulders
- May produce early and severe secondary osteoarthritis
- Pigment deposition may also occur in cardiac valves and genitourinary tissues, sometimes causing valvular disease or stones.
Diagnosis
- Raised homogentisic acid in urine
- Urine turns dark after standing or after alkalinization
- Molecular testing may identify pathogenic variants in HGD
Management
Treatment is largely supportive:
- Physiotherapy and analgesia for arthropathy
- Orthopedic intervention or joint replacement when required
- Nitisinone reduces homogentisic acid production, but raises tyrosine levels and requires monitoring.
One-line exam answer
Alkaptonuria is an autosomal recessive disorder caused by homogentisate oxidase deficiency, resulting in accumulation of homogentisic acid, blackening of urine on standing, and deposition of dark pigment in cartilage and connective tissue (ochronosis), leading to degenerative arthropathy.
A recent systematic review of nitisinone treatment found biochemical benefit, but treatment decisions still require monitoring for tyrosinemia and clinical outcomes (2025 systematic review, PMID 40157162).
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