Urea Cycle Enzyme Deficiencies and Clinical Findings
The urea cycle converts toxic ammonia into urea for renal excretion. Deficiencies in urea cycle enzymes cause hyperammonemia — a high-yield USMLE Step 1 topic. Each enzyme defect presents with distinct lab patterns and neurological symptoms that must be distinguished on boards.
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How to distinguish urea cycle defects
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What is the difference between OTC and CPS I deficiency?
OTC deficiency is X-linked and produces elevated orotic acid due to carbamoyl phosphate overflow into pyrimidine synthesis. CPS I deficiency is autosomal recessive with no orotic acid elevation.
- OTC: elevated orotic acid
- CPS I: normal orotic acid
- Both: elevated ammonia, low BUN
Why does hyperammonemia cause neurological symptoms?
Ammonia crosses the blood-brain barrier and reacts with alpha-ketoglutarate, depleting the TCA cycle. It also stimulates GABA-A receptors and increases cerebral edema via glutamine accumulation in astrocytes.
Which urea cycle disorder is X-linked?
OTC (ornithine transcarbamylase) deficiency is the only X-linked urea cycle disorder. Males present severely in the neonatal period; carrier females may have milder intermittent symptoms triggered by high-protein intake or illness.