This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Molecular Biology, Subject name: Biochemistry
A 4 year old girl presented to hospital with failure to thrive. Her peripheral blood smear showed megaloblastic anemia. Vitamin B12 & folate were given but anemia did not improve. Enzyme assay from the cultured PBMC showed deficiency of OP (Orotate Phospho Ribosyl Transferase). What could be the probable diagnosis ? 
Select and explain. 
A. Orotate deficiency, B. Gout, C. Orotic aciduria, D. SCID
C: Orotic aciduria
Answer: C: Orotic aciduria 
Explanation: Orotic aciduria Due to deficiency of orotate phosphoribosyl transferase and OMP decarboxylase Leads to defect in DNA Synthesis Characterizedby : Excretion of orotic acid in urine, Severe anemia Megaloblastic Not improved with folate /vitamin B12/vitamin C Retarded growth Rx- Feeding diet rich in uridine / cytidine