91Ó°ÊÓ

The Krebs cycle is a series of enzymatic reactions that catalyze the aerobic metabolism of fuel molecules to carbon dioxide and water, thereby generating energy for the production of ATP molecules. \(\alpha\)-ketoglutarate, succinate, and fumarate are intermediate products of this cycle. Therefore, their excess in the urine suggests an interruption or malfunction in this cycle.

Based on the information given, the specific enzyme must be involved in the conversion of \(\alpha\)-ketoglutarate to succinate and succinate to fumarate. Therefore, we have to look at the steps of the cycle which transform \(\alpha\)-ketoglutarate to succinate and then to fumarate. The enzyme involved after \(\alpha\)-ketoglutarate in the cycle is the \(\alpha\)-ketoglutarate dehydrogenase complex. This enzyme catalyzes the conversion of \(\alpha\)-ketoglutarate into succinyl-CoA. Then, succinyl-CoA is converted into succinate by succinyl-CoA synthetase. Lastly, succinate is converted into fumarate by the enzyme succinate dehydrogenase. Given that the excretion of all three, \(\alpha\)-ketoglutarate, succinate, and fumarate is increased, these compounds are not being converted into their subsequent metabolites within the cycle, meaning that the enzymatic dysfunction occurs after them in the cycle.

Hence, the enzyme that fits this description is fumarate hydratase, also known as fumarase. Fumarase is the enzyme that catalyzes the conversion of fumarate, the last mentioned compound in the problem, into malate within the Krebs cycle. This means that since fumarase is deficient, fumarate cannot be converted into malate and is instead excreted through urine, causing the symptoms mentioned in the problem.