Select the correct statement from the following.
A. The juxtamedullary nephrons have reduced Henle's loop.
B. Vasa recta is well developed in cortical nephrons.
C. The PCT and DCT are situated in the medulla of the kidney.
D. The ascending limb of the Henle's loop extends as the DCT.

Hint: DCT is the highly convoluted part of the renal tubule of a nephron. It extends from the ascending limb of the loop of Henle. DCT from many nephrons converges into the collecting duct.

Complete answer:
Let's discuss the basic structure of nephrons first. A nephron is the structural and functional unit of kidneys. Broadly, it has two parts: a glomerular capsule or Bowman’s capsule and a renal tubule.
Since the question is more about the renal tubules, let's discuss it only. A renal tubule has three main parts:
- Proximal convoluted tubule (PCT)
- Nephron loop (loop of Henle), and
- Distal convoluted tubule (DCT).

As the name suggests, PCT is the part of the renal tubule that is closer and attached to Bowman’s capsule. DCT, on the other hand, is located at the other end of the renal tubule. The Bowman’s capsule, PCT, and DCT lie in the renal cortex while the loop of Henle extends into the renal medulla.

Based mainly on the length of the loop of Henle, neurons are of two types: juxtamedullary nephrons and the cortical nephrons. The juxtamedullary nephrons have a long loop of Henle that extends deep into the renal medulla while the cortical nephrons have a shorter loop of Henle that extends only into the outer part of the renal medulla. Minute vessels from the peritubular capillaries supply the part of the renal tubules present in the renal medulla and form the vasa recta. Vasa recta is highly reduced in cortical nephrons.

The loop of Henle is a U-shaped structure that first extends from the renal cortex into the renal medulla, takes a U-turn, and then again enters into the renal cortex. The two arms of the U-shaped loop of Henle are called ascending and descending limbs.

The descending limb extends from the PCT and extends downwards into the medulla. The descending limb makes a hairpin turn in the medulla and extends upward into the renal cortex. This part of the loop of Henle is called the ascending limb of the loop of Henle as it ascends from the medulla to the renal cortex. The ascending limb of the nephron loop ends at the DCT. The DCT of several neurons drains to a single collecting duct.

Hence, The correct answer is option (D).

Note: renal cortex and renal medulla refer to the superficial and the deeper regions of a kidney. The collecting duct is not a structural part of a nephron but the nephrons are connected to the collecting duct through DCT.