Скачать или смотреть 25. Disorders of Sulfur-Containing Amino Acids | Homocystinuria, Cystathioninuria & Cystinuria USMLE

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Disorders of Sulfur-Containing Amino Acids | Homocystinuria, Cystathioninuria & Cystinuria | MBBS Biochemistry | USMLE Step 1

👋 Hey future doctors and biochem learners! Methionine and Cysteine are sulfur-containing amino acids crucial for methylation reactions, protein structure (disulfide bonds), and antioxidant defense. But when their metabolism is disrupted due to enzyme defects or transport defects, it leads to serious conditions collectively known as Disorders of Sulfur-Containing Amino Acids.

These conditions are high-yield exam topics because they integrate biochemistry, genetics, and clinical medicine, and are often tested in case-based questions in MBBS, NEET PG, FMGE, and USMLE Step 1. 🧬📚

🌟 Major Disorders of Sulfur-Containing Amino Acids

1️⃣ Homocystinuria

Defect: Most commonly due to Cystathionine β-synthase deficiency (B6-dependent).

Result: ↑ Homocysteine & Methionine, ↓ Cysteine.

Clinical 🩺:
• Marfanoid habitus (tall, long limbs).
• Downward lens dislocation (vs. Marfan → upward).
• Thrombosis & early atherosclerosis → stroke, MI.
• Osteoporosis, developmental delay.

Exam clue: “Young patient with stroke + lens dislocation.”

Treatment: Pyridoxine (B6), folate, B12, low methionine diet, betaine.

2️⃣ Cystathioninuria

Defect: Cystathionase deficiency.

Result: ↑ Cystathionine in urine.

Clinical 🩺: Usually benign, sometimes mild neurodevelopmental symptoms.

3️⃣ Cystinuria (actually a transport disorder, not enzyme defect)

Defect: Renal tubular reabsorption of COLA amino acids (Cystine, Ornithine, Lysine, Arginine).

Result: Excess cystine in urine → recurrent kidney stones (hexagonal crystals).

Clinical 🩺: Flank pain, hematuria, renal stones in young age.

Exam clue: “Hexagonal crystals in urine.”

Treatment: Hydration, urinary alkalinization, chelating agents (penicillamine).

🧪 Diagnosis

Plasma amino acid analysis.

Urine chromatography or aminoaciduria patterns.

Homocystinuria → ↑ homocysteine in plasma/urine.

Cystinuria → cyanide-nitroprusside test positive.

⚕️ Exam Integration

Homocystinuria = lens dislocation (downward), thrombosis, marfanoid, osteoporosis.

Cystinuria = recurrent stones, hexagonal crystals, “COLA defect.”

Cofactors like Vitamin B6, Folate, B12 often appear in exam questions.
👉 A favorite biochemistry + clinical vignette topic for USMLE Step 1 and NEET PG.

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