Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Relative thickness of the different limbs (the thin descending limb, thin ascending limb and thick ascending limb) of loop of Henle is different but lumen diameter is same.
Reason (R) : Difference in relative thickness is due to differences in the thickness of epithelial cells but does not refer to change in lumen diameter.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Remember that in renal histology, "thick" and "thin" refer to the height of the epithelial lining cells (cuboidal vs. squamous), not the width of the tubule's internal lumen.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The Loop of Henle is a U-shaped portion of the nephron in the kidney.
It is divided into a thin descending limb, a thin ascending limb, and a thick ascending limb based on histological differences.

Step 2: Detailed Explanation:
Assertion (A) states that the outer thickness of the different segments of the Loop of Henle varies, while the internal lumen diameter remains relatively uniform.
This is anatomically correct.
The distinction between "thin" and "thick" limbs does not mean the internal channel (lumen) gets wider or narrower.
Instead, as stated in Reason (R), this difference is due to the type of epithelial cells lining the tubule walls:
- The thin segments are lined by flat, thin squamous epithelial cells.
- The thick segment is lined by larger, low cuboidal epithelial cells containing many mitochondria.
Because the cuboidal cells are much taller than the flat squamous cells, the outer wall of the thick segment is thicker, though the inner lumen diameter remains similar.
Therefore, the difference in outer wall thickness is due to the height of the lining cells, not a change in the lumen.
Since Assertion (A) is true, Reason (R) is true, and (R) correctly explains why (A) occurs, they are closely linked.

Step 3: Final Answer:
Both (A) and (R) are true and (R) is the correct explanation of (A).