If a patient has a biochemical block in the formation of urea, which substance will accumulate in the blood?

In conditions where there is a biochemical block in the formation of urea, the substance that will accumulate in the blood is ammonia. Urea is produced in the liver through the urea cycle, a process that transforms toxic ammonia, which is a byproduct of protein metabolism, into urea for safe excretion through the urine.

When there is a disruption in this process, such as in certain genetic disorders or liver diseases affecting the urea cycle, ammonia cannot be converted efficiently into urea, leading to elevated levels of ammonia in the bloodstream. This accumulation of ammonia can be toxic and cause symptoms such as confusion or lethargy, often referred to as hepatic encephalopathy if related to liver dysfunction.

The other substances listed, while important in metabolism, do not specifically accumulate due to a block in urea formation. Creatinine is a waste product of muscle metabolism and is typically used as an indicator of kidney function rather than a direct product of the urea cycle. Glucose is a primary energy source for the body, and its levels would not be directly influenced by dysfunction in the urea cycle. Therefore, ammonia is the correct answer as it is the direct consequence of the impaired metabolic pathway.