This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
PRPP paicipates in purine biosynthesis in _____ 
Select and explain. 
A. Serves as a scaffold for assembly of purine ring, B. Post assembly of purine ring, C. Both, D. None
A: Serves as a scaffold for assembly of purine ring
Answer: A: Serves as a scaffold for assembly of purine ring 
Explanation: PRPP serves as a scaffold for assembly of the purine ring in purine biosynthesis. Phosphoribosyl pyrophosphate (PRPP) paicipates in pyrimidine biosynthesis only subsequent to assembly of the pyrimidine ring. Reference: Harpers illustrated biochemistry 30th edition