**Core Concept:** Nephrons are the functional units of the kidney that filter blood and produce urine. They consist of several parts, including the glomerulus and the renal tubules. The glomerulus forms a filtration barrier due to the presence of three types of capillaries and the Bowman's capsule. The renal tubules further process and reabsorb essential substances from the filtrate.

**Why the Correct Answer is Right:** Option D, the juxtaglomerular apparatus, is not a part of the filtration barrier in nephrons. It is located at the entrance of the renal tubules and plays a role in blood pressure regulation and renin secretion.

**Why Each Wrong Option is Incorrect:**

A. The glomerulus forms the initial filtration barrier due to the dense network of capillaries and the Bowman's capsule.
B. The renal tubules, composed of proximal convoluted tubules, loop of Henle, and distal convoluted tubules, contribute to reabsorption and secretion processes but not filtration.
C. The renal corpuscle, which includes the glomerulus and Bowman's capsule, is responsible for filtration in nephrons.

**Clinical Pearl:** Understanding the filtration barrier in nephrons is crucial for understanding kidney function, electrolyte balance, and disorders like nephrotic syndrome and glomerulonephritis. A thorough knowledge of nephrology is essential for practicing physicians treating patients with kidney-related diseases.

**Correct Answer:** C. Renal corpuscle (glomerulus and Bowman's capsule) does not form the filtration barrier in nephrons.