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
In the citric acid cycle, the reaction which produces succinyl CoA also forms carbon dioxide. The key intermediate enzymic step involves: 
Select and explain. 
A. The decarboxylation of a hydroxy acid, B. The decarboxylation of an alpha-ketoacid, C. The decarboxylation of a beta-ketoacid, D. The oxidation of a keto group to a carboxylic acid
B: The decarboxylation of an alpha-ketoacid
Answer: B: The decarboxylation of an alpha-ketoacid 
Explanation: The enzyme is a-ketoglutarate dehydrogenase, an a-keto acid, which is decarboxylated. Therefore the second choice is correct. The only confusion might be with the last choice, since in effect it involves the oxidation of a keto group to a carboxylic acid. However, the product is the thio-ester of a carboxylic acid.