First pyrimidine nucleotide to be synthesized is :

First pyrimidine nucleotide to be synthesized OMP (Orotidine Mono Phosphate). First purine nucleotide to be synthesized IMP (Inosine Mono Phosphate). Fig- Simplified Pyrimidine biosynthesis pathway Note: Chemically Orotidine monophosphate is also a pyrimidine nucleotide though it is not present in DNA or RNA. The question just asked you about "pyrimidine nucleotide" and not "pyrimidine nucleotide present in DNA or RNA". That why the answer is OMP not UMP.