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?

The correct answer to the question "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?" is Option B: Uridine monophosphate synthase.

The other options — A) Dihydrofolate reductase, C) Methylmalonyl-CoA mutase, D) Adenosine deaminase — are incorrect for this USMLE Exam Prep question. As explained above: 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…