**Core Concept**
The proximal convoluted tubule (PCT) is a crucial part of the nephron in the kidneys, responsible for reabsorbing approximately 2/3 of the filtered glucose, amino acids, and ions back into the bloodstream. This reabsorption is primarily driven by active transport mechanisms, involving specific transport proteins and ion channels.

**Why the Correct Answer is Right**
The correct answer is sodium (Na+). Sodium reabsorption in the PCT is facilitated by the Na+/glucose cotransporter (SGLT2) and the Na+/phosphate cotransporter (NPT2). This reabsorption is essential for maintaining electrolyte balance and water reabsorption in the kidneys. As sodium ions are reabsorbed, water follows passively due to osmosis, contributing to the overall fluid reabsorption in the PCT.

**Why Each Wrong Option is Incorrect**
**Option A:** Potassium ions (K+) are primarily reabsorbed in the distal convoluted tubule and collecting ducts, not in the PCT.

**Option B:** Calcium ions (Ca2+) are reabsorbed in multiple segments of the nephron, including the PCT, but the primary reabsorption mechanism in the PCT is not the correct answer.

**Option D:** There is no significant reabsorption of chloride ions (Cl-) in the PCT. Chloride reabsorption primarily occurs in the distal convoluted tubule and collecting ducts.

**Clinical Pearl / High-Yield Fact**
The PCT is responsible for reabsorbing approximately 2/3 of the filtered glucose, making it a crucial site for glucose homeostasis in the body. Impaired glucose reabsorption in the PCT can lead to glucose loss in the urine, resulting in glycosuria.

**Correct Answer:** C. Sodium (Na+)