OTC deficiency vs Other urea cycle defects
How to tell OTC deficiency and Other urea cycle defects apart when a board question makes them look alike.
OTC is hyperammonemia with high orotic acid and no megaloblastic anemia; other urea defects separate by citrulline/argininosuccinate/arginine patterns.
Side by side
OTC deficiency causes hyperammonemia, respiratory alkalosis, vomiting, lethargy, low BUN, high glutamine, and elevated orotic acid without megaloblastic anemia.
Other urea cycle defects also cause hyperammonemia but may lack high orotic acid or differ by citrulline/argininosuccinate/arginine patterns.
X-linked ornithine transcarbamylase deficiency prevents carbamoyl phosphate entry into the urea cycle, shunting it to pyrimidine synthesis.
Defects in CPS1, ASS1, ASL, arginase, or NAGS block other steps of ammonia detoxification and create step-specific amino acid patterns.
Hyperammonemia plus high orotic acid, low citrulline, and no megaloblastic anemia support OTC deficiency.
Very low citrulline without orotic acid suggests CPS1/NAGS; high citrulline suggests ASS1; high argininosuccinate suggests ASL.
The trap
High orotic acid plus hyperammonemia points to OTC; high orotic acid plus megaloblastic anemia without hyperammonemia points to UMP synthase deficiency.
The repair move
For urea cycle disease, combine ammonia level with orotic acid and citrulline pattern.
Try a free question
CoreA 3-day-old boy develops poor feeding, vomiting, lethargy, hyperventilation with respiratory alkalosis, and seizures. Plasma ammonia is 480 mcmol/L (markedly elevated), BUN is low, and urine orotic acid is markedly elevated. There is no megaloblastic anemia. Family history reveals a maternal uncle who died of unexplained encephalopathy as a neonate. Which enzyme is deficient?
Show answer and explanation
Answer: A. Ornithine transcarbamylase (OTC deficiency, X-linked). OTC deficiency is X-linked; carbamoyl phosphate cannot enter the urea cycle and is shunted to pyrimidine synthesis, generating orotic acid. Hyperammonemia plus elevated orotic acid WITHOUT megaloblastic anemia and X-linked male predominance distinguishes OTC.
Why the other choices are tempting
- B. CPS1 deficiency lacks elevated orotic acid because the block is upstream of orotic acid generation.
- C. ASS1 deficiency (citrullinemia) shows elevated citrulline rather than the pattern here.
- D. ASL deficiency shows elevated argininosuccinate, not isolated orotic acid elevation without that intermediate.
- E. UMP synthase deficiency causes orotic aciduria WITH megaloblastic anemia and NO hyperammonemia, the opposite pattern.
Flashcards
Tap a card to flip it.
What clue supports OTC deficiency?
Choose OTC deficiency when the stem gives this pattern: OTC deficiency causes hyperammonemia, respiratory alkalosis, vomiting, lethargy, low BUN, high glutamine, and elevated orotic acid without megaloblastic anemia.
What clue supports Other urea cycle defects?
Choose Other urea cycle defects when the stem gives this contrasting pattern: Other urea cycle defects also cause hyperammonemia but may lack high orotic acid or differ by citrulline/argininosuccinate/arginine patterns.
Quick answers
What is the fastest way to tell OTC deficiency from Other urea cycle defects?
OTC is hyperammonemia with high orotic acid and no megaloblastic anemia; other urea defects separate by citrulline/argininosuccinate/arginine patterns.
What is the key clue for OTC deficiency?
OTC deficiency causes hyperammonemia, respiratory alkalosis, vomiting, lethargy, low BUN, high glutamine, and elevated orotic acid without megaloblastic anemia.
What is the key clue for Other urea cycle defects?
Other urea cycle defects also cause hyperammonemia but may lack high orotic acid or differ by citrulline/argininosuccinate/arginine patterns.
What is the common trap with OTC deficiency vs Other urea cycle defects?
High orotic acid plus hyperammonemia points to OTC; high orotic acid plus megaloblastic anemia without hyperammonemia points to UMP synthase deficiency.