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: Biochemistry
Expected change in Vitamin B,2 deficiency 
Select and explain. 
A. Increased homocysteine in urine, B. Increased methylmalonic acid in urine, C. FIGLU in urine, D. Phenylalanine in urine
B: Increased methylmalonic acid in urine
Answer: B: Increased methylmalonic acid in urine 
Explanation: Ans. is 'b' i.e., Increased methylmalonic acid in urine Isomerization of methvlmalonyl CoA to succinyl CoAIn this reaction, active form of vitamin BI2 is deoxyadenosyl cobalamine.Propionyl-CoA is produced as catabolic end product of some alipathic amino acids andb-oxidation of odd chain fatty acids.Propionyl CoA is then conveed to succinyl CoA through methylmalonyl-CoA.Thus methylmalonyl-CoA is accumulated and excreted in urine as methylmalonic acid (methylmalonate) in vitamin B12 deficiency, i.e. methylmalonic aciduria.