Скачать или смотреть 21. Disorders of Aromatic Amino Acid Metabolism | Phenylketonuria (PKU) | Biochemistry | USMLE

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Disorders of Aromatic Amino Acid Metabolism | Phenylketonuria (PKU) | Medical Biochemistry | MBBS | USMLE Step 1

👋 Hey future doctors and biochem learners! Did you know that a single enzyme defect in the metabolism of phenylalanine, an aromatic amino acid, can cause one of the most common inborn errors of metabolism? This condition is called Phenylketonuria (PKU) — a classic, high-yield disorder tested in MBBS, NEET PG, FMGE, and USMLE Step 1.

In this lecture, we’ll break down the biochemical pathway, enzyme defect, clinical features, and management of PKU, integrating basic science with clinical medicine. 🧬📚

🌟 Biochemical Basis of PKU

Normal Pathway:
• Phenylalanine → Tyrosine (enzyme: Phenylalanine hydroxylase, requires BH₄ cofactor).

In PKU:
• Deficiency of Phenylalanine hydroxylase (most common) or BH₄ deficiency →
• ↑ Phenylalanine, ↓ Tyrosine.
• Phenylalanine converted to alternative metabolites (phenylpyruvate, phenyllactate, phenylacetate) → toxic to brain.

🧪 Clinical Features

Mental retardation (intellectual disability if untreated).

Seizures, developmental delay.

Hypopigmentation (↓ melanin, since tyrosine → melanin pathway blocked).

“Mousy” or musty odor of urine (phenylacetate).

Eczema-like skin changes.

🩺 Types of PKU
Classical PKU: Phenylalanine hydroxylase deficiency.
Malignant PKU: Defect in BH₄ metabolism (also causes ↓ dopamine, serotonin).

⚕️ Diagnosis & Screening
Newborn screening: Guthrie bacterial inhibition assay, tandem mass spectrometry.
↑ Plasma phenylalanine levels.

⚙️ Treatment
Dietary restriction of phenylalanine (low-protein diet).
Supplement tyrosine (becomes essential in PKU).
Avoid aspartame (contains phenylalanine).
BH₄ supplementation in malignant PKU.

🎯 Exam Integration

PKU is the prototype of aromatic amino acid metabolism disorders.

Remember: Phenylalanine ↑, Tyrosine ↓, Musty odor urine, Intellectual disability if untreated.
👉 A favorite question in both recall and clinical case format in USMLE & NEET PG.

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