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Inborn Errors of Amino Acid Metabolism | Disorders, Clinical Features & Biochemistry | MBBS Biochemistry | USMLE Step 1

👋 Hey future doctors and biochem learners! Amino acids are vital for life — but when their metabolic pathways go wrong due to enzyme deficiencies, the result is a group of conditions called Inborn Errors of Amino Acid Metabolism. 🧬⚠️ These are classic exam topics because they link genetic mutations, biochemical blocks, metabolite accumulation, and clinical presentations.

This lecture provides an overview of the major amino acid disorders, their biochemical basis, and their clinical features — essential for MBBS, NEET PG, FMGE, and USMLE Step 1.

🌟 Major Inborn Errors of Amino Acid Metabolism

1️⃣ Phenylketonuria (PKU)

Deficiency: Phenylalanine hydroxylase or BH₄ cofactor.

Result: ↑ Phenylalanine, ↓ Tyrosine.

Clinical 🩺: Intellectual disability, seizures, hypopigmentation, musty odor.

2️⃣ Alkaptonuria

Deficiency: Homogentisate oxidase (tyrosine metabolism).

Result: ↑ Homogentisic acid.

Clinical 🩺: Dark urine, ochronosis, arthritis.

3️⃣ Maple Syrup Urine Disease (MSUD)

Deficiency: Branched-chain α-ketoacid dehydrogenase.

Result: ↑ Leucine, Isoleucine, Valine.

Clinical 🩺: Sweet-smelling urine, vomiting, seizures, neurological decline.

4️⃣ Homocystinuria

Deficiency: Cystathionine β-synthase (B6-dependent) or defects in homocysteine metabolism.

Result: ↑ Homocysteine & Methionine.

Clinical 🩺: Marfanoid habitus, lens dislocation, thrombosis, osteoporosis.

5️⃣ Tyrosinemia (Type I, II, III)

Enzyme defects in tyrosine catabolism.

Clinical 🩺: Liver failure, renal tubular dysfunction, neurologic issues.

6️⃣ Hartnup Disease

Defect in intestinal/renal transport of neutral amino acids (esp. tryptophan).

Result: ↓ Tryptophan → ↓ Niacin → Pellagra-like symptoms (dermatitis, diarrhea, dementia).

7️⃣ Cystinuria (transport disorder)

Defect in renal transport of cystine, ornithine, lysine, arginine (COLA).

Clinical 🩺: Recurrent kidney stones.

🩺 Clinical Integration

Most disorders present in infancy or childhood with failure to thrive, neurologic symptoms, unusual urine odor/color.

Early diagnosis via newborn screening prevents complications (e.g., Guthrie test for PKU).

Dietary management is key: restrict offending amino acid, supplement downstream product or cofactor.

🎯 Exam Integration

Know enzyme defect → accumulating metabolite → clinical features → lab findings.

Mnemonics help:
• PKU = “Phenylalanine builds up”
• MSUD = “I Love Vermont” (Ile, Leu, Val)
• Homocystinuria = “HOMOCY” = Homocysteine ↑, Osteoporosis, Marfanoid, Ocular lens dislocation, Cardiovascular clots, Young stroke.
👉 These appear in both recall and case-based questions in USMLE & NEET PG.

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❓Which disorder should we cover in detail next — PKU, MSUD, or Homocystinuria? Drop your request in the comments below! 😊

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