A 1-year-old infant presents with megaloblastic anemia, developmental delay, and failure to thrive. Laboratory tests reveal elevated orotic acid in the urine. The megaloblastic anemia does not respond to vitamin B12 or folate supplementation. Which enzyme deficiency is most likely responsible for this condition?
✓ Correct Answer: Option B
Uridine monophosphate (UMP) synthase deficiency causes orotic aciduria, impaired pyrimidine synthesis, and megaloblastic anemia refractory to B12/folate. Orotic acid accumulates as an intermediate in pyrimidine synthesis.