This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
The genetic variation in drug metabolism pathway that results in severe toxicity of fluorouracil is: 
Select and explain. 
A. CYP2C9, B. Dihydropyrimidine dehydrogenase, C. Thiopurine-S methyltransferase, D. CYP2D6
B: Dihydropyrimidine dehydrogenase
Answer: B: Dihydropyrimidine dehydrogenase 
Explanation: Genetic variation in drug metabolism pathway dihydropyrimidine dehydrogenase results in severe toxicity of capecitabine, fluorouracil. Ref: Harrison, Edition-18, Page-42.