Скачать или смотреть 22. Disorders of Aromatic Amino Acid Metabolism | Alkaptonuria | Medical Biochemistry | MBBS | USMLE

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Disorders of Aromatic Amino Acid Metabolism | Alkaptonuria | Medical Biochemistry | MBBS | USMLE Step 1

👋 Hey future doctors and biochem enthusiasts! Have you ever heard of urine that turns black on standing? 🖤 That’s the hallmark of Alkaptonuria, a rare inborn error of tyrosine metabolism. It’s one of the oldest known metabolic disorders, first described by Sir Archibald Garrod in his concept of “inborn errors of metabolism.”

This lecture explains the pathway, enzyme defect, clinical features, and exam relevance of Alkaptonuria — a must-know topic for MBBS, NEET PG, FMGE, and USMLE Step 1. 🧬📚

🌟 Biochemical Basis of Alkaptonuria

Normal Pathway:
• Tyrosine → Homogentisic acid → Maleylacetoacetate (enzyme: Homogentisate oxidase).

In Alkaptonuria:
• Deficiency of Homogentisate oxidase → accumulation of homogentisic acid.
• Homogentisic acid is excreted in urine and deposited in connective tissues.

🧪 Clinical Features

Black urine: homogentisic acid oxidizes on standing, turning urine dark.

Ochronosis: bluish-black pigmentation in cartilage, sclera, and skin due to tissue deposition.

Arthropathy: degeneration of cartilage → early-onset arthritis (especially spine and large joints).

Symptoms appear late in life despite congenital defect.

🩺 Diagnosis

Clinical: blackening of urine on standing.

Lab: elevated homogentisic acid in urine (chromatography, mass spectrometry).

⚙️ Treatment

No complete cure.

Dietary restriction: low protein, especially phenylalanine & tyrosine.

Nitisinone: inhibits homogentisate formation (newer therapy).

Vitamin C: may reduce pigment deposition.

Supportive: pain management for arthritis.

🎯 Exam Integration

Enzyme defect: Homogentisate oxidase.

Key features: black urine, ochronosis, arthritis.

Mnemonic: “Alkaptonuria = Arthritis + Black urine + Cartilage pigmentation (ochronosis)”.
👉 A favorite short case question in USMLE & NEET PG.

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❓Should we cover Tyrosinemias (Types I, II, III) as the next aromatic amino acid disorders in your playlist? Drop a comment below! 😊

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