Скачать или смотреть 24. Disorders of Branched Chain Amino Acid Metabolism | MSUD, Isovaleric & Propionic Acidemia USMLE

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Disorders of Branched Chain Amino Acid Metabolism | MSUD, Isovaleric & Propionic Acidemia | MBBS Biochemistry | USMLE Step 1

👋 Hey future doctors and biochem learners! The branched-chain amino acids — Valine, Leucine, and Isoleucine — are essential for growth and energy metabolism. But when their catabolic pathways are blocked due to enzyme defects, the result is a group of serious conditions called Disorders of Branched-Chain Amino Acid Metabolism.

These are classic inborn errors of metabolism and favorite exam questions in MBBS, NEET PG, FMGE, and USMLE Step 1, often presented as newborns with feeding problems, seizures, metabolic acidosis, and unusual urine odors. 🧬📚

🌟 Major Disorders of BCAA Metabolism

1️⃣ Maple Syrup Urine Disease (MSUD)

Defect: Branched-chain α-ketoacid dehydrogenase (BCKD).

Accumulation: Leu, Ile, Val and their ketoacids.

Clinical 🩺: Poor feeding, vomiting, lethargy, seizures, developmental delay, and sweet “maple syrup” odor in urine.

Exam clue: “Newborn with seizures and urine that smells sweet.”

2️⃣ Isovaleric Acidemia

Defect: Isovaleryl-CoA dehydrogenase (Leucine metabolism).

Clinical 🩺: Metabolic acidosis, vomiting, lethargy, “sweaty feet” odor of urine.

3️⃣ Propionic Acidemia

Defect: Propionyl-CoA carboxylase (Val, Ile, Thr, Met metabolism).

Clinical 🩺: Severe metabolic acidosis, hyperammonemia, vomiting, developmental delay.

4️⃣ Methylmalonic Acidemia

Defect: Methylmalonyl-CoA mutase or Vitamin B12 deficiency.

Clinical 🩺: Similar to propionic acidemia with metabolic acidosis and neurologic symptoms; ↑ methylmalonic acid in urine.

🧪 Diagnosis

Newborn screening: tandem mass spectrometry.

Plasma amino acid and organic acid analysis.

Enzyme assays and genetic testing.

⚕️ Management

Dietary restriction of branched-chain amino acids.

Thiamine supplementation (some MSUD variants are thiamine-responsive).

Special metabolic formulas.

Supportive care for acidosis (bicarbonate, carnitine).

Vitamin B12 supplementation in methylmalonic acidemia.

🎯 Exam Integration

MSUD = sweet urine smell (Leu, Ile, Val defect).

Isovaleric acidemia = sweaty feet odor.

Propionic vs. Methylmalonic acidemia = metabolic acidosis with hyperammonemia.
👉 Mnemonic: "I Love Vermont" (Ile, Leu, Val) = branched-chain amino acids.

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