Which reaction of the urea cycle occurs in the mitochondria?

The correct choice is the condensation of ornithine and carbamoyl phosphate, as it is the first step of the urea cycle that takes place in the mitochondria. During this reaction, ornithine reacts with carbamoyl phosphate to form citrulline, facilitated by the enzyme ornithine transcarbamylase. This mitochondrial reaction is crucial because it initiates the urea cycle, a vital pathway for detoxifying ammonia in the liver.

The subsequent reactions of the urea cycle occur in the cytosol, which is why the other options are not correct. The conversion of citrulline to arginine happens in the cytosol, as does the conversion of arginine to ornithine, which is catalyzed by arginase in the final step of the cycle. The hydrolysis of urea into ammonia is not part of the urea cycle itself; rather, urea is excreted as a waste product of the cycle. Therefore, option B accurately represents the reaction that occurs in the mitochondria, highlighting its role in the urea cycle's function.