Question

The principal nitrogenous constituent of mammalian urine is

• A. Amino acids
• B. Uric acid
• C. Urea
• D. Ammonia

Hint:

Dalmation dogs are a unique mammalian exception; due to a genetic defect in uric acid transport into hepatocytes, they excrete high levels of uric acid in their urine instead of converting it to allantoin.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Animals excrete excess nitrogen produced from the catabolism of amino acids and nucleic acids in different chemical forms.
Based on their primary nitrogenous waste product, animals are classified as ammonotelic (excreting ammonia), uricotelic (excreting uric acid), or ureotelic (excreting urea).

Step 2: Detailed Explanation:
Let us compare these nitrogenous waste products:
Ammonia: Highly toxic and requires large volumes of water to dilute and excrete safely. It is the primary waste product of aquatic animals (ammonotelic).
Uric Acid: Relatively non-toxic and insoluble in water, allowing it to be excreted as a semi-solid paste with minimal water loss. It is the primary waste product of birds and reptiles (uricotelic).
Urea: Mammals are ureotelic organisms.
The catabolism of amino acids generates highly toxic free ammonia (\(\text{NH}_3\)).
To prevent systemic toxicity, hepatocytes in the liver rapidly convert ammonia and bicarbonate into urea via the ornithine-urea cycle (urea cycle).
Urea is highly soluble, relatively non-toxic, and is transported in the blood to the kidneys, where it is filtered and excreted.
Therefore, urea is the principal nitrogenous constituent of mammalian urine, accounting for over \(80\%\) of total urinary nitrogen.

Step 3: Final Answer:
The principal nitrogenous constituent of mammalian urine is urea.