Select the true statement -

Options

A. In fishes kidney play a major role in ammonia excretion.
B. Ammonia is 100,000 times less toxic than urea.
C. Sharks retain a large amount of urea in the blood as a major osmolyte to balance the osmolarity of the body fluids
D. Most terrestrial reptile excrete ammonia

Answer

To solve the question, we need to evaluate each statement provided and determine which one is true.

**Step 1: Analyze the first statement**
- The first statement claims that in fishes, the kidney plays a major role in ammonia excretion.
- In reality, fishes primarily excrete ammonia directly through their gills rather than relying heavily on their kidneys. Therefore, this statement is false.

**Step 2: Analyze the second statement**
- The second statement claims that ammonia is 100,000 times less toxic than urea.
- This is incorrect as ammonia is actually more toxic than urea. Urea is less toxic and is a more stable compound for excretion in terrestrial animals. Thus, this statement is also false.

**Step 3: Analyze the third statement**
- The third statement mentions that sharks retain a large amount of urea in their blood as a major osmolyte to balance the osmolarity of body fluids.
- This statement is true. Sharks, being marine creatures, do retain urea in their blood to maintain osmotic balance with the surrounding seawater, preventing dehydration.

**Step 4: Analyze the fourth statement**
- The fourth statement suggests that most terrestrial reptiles excrete ammonia.
- This is false. Most terrestrial reptiles excrete uric acid, which is less toxic and conserves water, making it suitable for their environment.

**Conclusion:**
The true statement among the options provided is the third statement: "Sharks retain a large amount of urea in blood as a major osmolyte to balance the osmolarity of the body fluids."

---