Скачать или смотреть 19. Urea Cycle Disorders | Enzyme Defects, Hyperammonemia & Clinical Features | USMLE Step 1

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Urea Cycle Disorders | Enzyme Defects, Hyperammonemia & Clinical Features | Medical Biochemistry | USMLE Step 1 MBBS

👋 Hey future doctors and biochem learners! Did you know that defects in the urea cycle enzymes prevent safe disposal of nitrogen, leading to hyperammonemia — a life-threatening condition? 🧠☠️ These Urea Cycle Disorders (UCDs) are classic exam topics that link biochemistry, pediatrics, and clinical medicine, making them must-know for MBBS, NEET PG, FMGE, and USMLE Step 1.

This lecture explains the types, clinical features, lab findings, and management of urea cycle disorders in a simple, exam-focused way. 🧬📚

🌟 Major Urea Cycle Disorders

1️⃣ Carbamoyl Phosphate Synthetase I (CPS-I) Deficiency

Severe hyperammonemia soon after birth.

No orotic acid elevation.

2️⃣ Ornithine Transcarbamylase (OTC) Deficiency ❗

Most common UCD, X-linked.

Hyperammonemia + ↑ orotic acid in urine (due to carbamoyl phosphate shunting into pyrimidine synthesis).

3️⃣ Argininosuccinate Synthetase Deficiency (Citrullinemia)

↑ Citrulline in plasma/urine.

4️⃣ Argininosuccinate Lyase Deficiency

↑ Argininosuccinate in plasma/urine.

Symptoms: hepatomegaly, brittle hair.

5️⃣ Arginase Deficiency

↑ Arginine in plasma.

Symptoms: spasticity, neurologic dysfunction, milder hyperammonemia.

🧪 Clinical Features of UCDs

Neonatal onset: vomiting, poor feeding, lethargy, seizures, cerebral edema.
Late onset (partial defects): episodic vomiting, confusion, protein intolerance.
Lab clues: very high plasma ammonia, metabolic alkalosis, specific metabolite accumulation (depending on enzyme defect).

⚕️ Management Strategies
Dietary: Protein restriction (but adequate for growth).
Medications: Nitrogen scavengers (sodium benzoate, sodium phenylbutyrate).
Supplementation: Arginine (in some defects), citrulline (in OTC deficiency).
Definitive: Liver transplantation in severe cases.

🎯 Exam Integration
Always identify the enzyme defect → metabolite accumulation → lab marker.
OTC deficiency = hyperammonemia + orotic acid ↑.
CPS-I deficiency = hyperammonemia + no orotic acid.
👉 These are favorite case-based questions in USMLE Step 1 & NEET PG.
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❓Want us to cover Hyperammonemia (primary vs secondary) and its detailed management as a separate video? Drop your request in the comments below! 😊

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