Question

A patient is placed on a restricted diet of water, pure cooked starch, olive oil, adequate minerals, and vitamins. If a urinalysis several weeks later reveals the presence of relatively normal amounts of urea, the urea probably came from the?

1) food eaten during the restricted diet
2) withdrawal of reserve urea stored in the liver
3) chemical combination of water, carbon dioxide, and free nitrogen
4) deamination of cellular proteins
5) urea synthesized by kidney tubule cells

Answer 1

The urea found in the patient's urine likely came from the deamination of cellular proteins, as the restricted diet was low in protein and urea synthesis is a natural part of protein metabolism and turnover in the body.

Step-by-step explanation:

The presence of relatively normal amounts of urea in the patient's urine despite being on a restricted diet suggests that the urea was produced endogenously. Urea is a nitrogen-containing molecule that is synthesized in the liver from ammonia, which is released by the oxidation of amino acids. This process is part of the body's natural metabolism, where proteins from cellular turnover are broken down, and their amino groups (deamination) are removed to form ammonia. This ammonia is then rapidly converted into urea in the liver because ammonia is toxic. The liver does not store reserve urea, so option 2 is incorrect. As the patient's diet is essentially free of protein, de novo synthesis of urea from water, carbon dioxide, and free nitrogen (option 3) is biochemically implausible and kidney tubule cells do not synthesize urea (option 5).

Since the deamination of cellular proteins is an ongoing process to regulate amino acid levels and remove excess nitrogen, it is the most likely source of urea found in the patient's urine. Option 4, the deamination of cellular proteins, is the correct answer because even without dietary protein, body proteins will continually turn over, and their breakdown results in the production of urea.