The kidneys secrete ADH in response to dehydration.

A. Azotemia is a condition characterized by elevated levels of nitrogenous waste products in the blood, not a direct by-product of protein catabolism.

B. Creatinine is a by-product of muscle metabolism, not directly from protein catabolism.

C. Uric acid is a by-product of nucleic acid metabolism, not protein catabolism.

D. Urea is a primary by-product of protein catabolism, formed in the liver and excreted by the kidneys. This is the correct answer.

A. Metabolic water; cutaneous transpiration: Metabolic water refers to the water produced during metabolic processes within the body. However, most body water intake comes from drinking fluids, not metabolic water.

B. Drinking; cutaneous transpiration and expired air: While drinking is indeed the main source of water intake, the greatest loss of water from the body is through urine, not cutaneous transpiration and expired air.

C. Drinking; radiation: While drinking is the primary source of water intake, radiation is not a significant way the body loses water.

D. Drinking; urine: Drinking fluids is the main source of water intake, and urine excretion is the primary way the body loses water.