Which tubule segment would be labeled as PCT on a diagram of the nephron?

• A. DCT
• B. PCT
• C. Collecting duct
• D. Loop of Henle

Answer: (B)

The main idea here is recognizing the proximal tubule on a nephron diagram by its distinctive structure and position. The proximal tubule is the first tubular segment that emerges from Bowman's capsule and sits in the renal cortex. It is lined by tall simple cuboidal epithelial cells with a dense brush border of microvilli, which dramatically increases surface area for reabsorption. That high reabsorptive capacity explains why this segment is depicted with such a prominent brush border and why it’s the region you’d label as the proximal tubule. The cells’ abundant mitochondria support active transport of solutes and water, consistent with the proximal tubule’s role in reclaiming most of the filtered Na+, bicarbonate, glucose, amino acids, and water. In contrast, other segments—like the loop of Henle, distal convoluted tubule, and collecting duct—have different cell shapes and surface features (fewer microvilli, different permeability and transport profiles), so they wouldn’t be labeled as the proximal tubule.