This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nucleotides, Subject name: Biochemistry
Deficiency of which of the following enzyme leads to toxicity of 5-fluorouracil? 
Select and explain. 
A. Purine nucleoside phosphorylase, B. Dihydropyrimidine dehydrogenase, C. PRPP synthase, D. Adenosine deaminase
B: Dihydropyrimidine dehydrogenase
Answer: B: Dihydropyrimidine dehydrogenase 
Explanation: 5-fluorouracil is acted upon by dihydropyrimidine dehydrogenase - Deficiency - accumulation of 5-FU - toxicity.