Which compound is the most abundant non-protein nitrogen (NPN) substance?

• A. Urea
• B. Amino acids
• C. Uric acids
• D. Creatinine

Answer: (A)

The main idea is that the body disposes of excess nitrogen primarily by forming urea, a soluble end product of the urea cycle in the liver. Urea is then carried to the kidneys for excretion, so it becomes the dominant non-protein nitrogen (NPN) compound found in blood and urine. Among the options, urea stands out because it is produced specifically to safely remove ammonia, the toxic nitrogen form from amino acid and protein breakdown, and it is present in much higher amounts than other NPN substances. Free amino acids exist only in relatively small pools, while creatinine and uric acid come from muscle metabolism and purine breakdown respectively and appear in lower concentrations. Therefore, urea is the most abundant NPN substance.