**Core Concept**
The mechanism of water reabsorption in the proximal convoluted tubule (PCT) is a critical process that helps regulate the body's fluid balance and electrolyte levels. This process involves the reabsorption of water and electrolytes, primarily sodium, from the filtrate back into the bloodstream.

**Why the Correct Answer is Right**
Water reabsorption in the PCT is primarily driven by the reabsorption of sodium ions (Na+), which is facilitated by the sodium-potassium pump (Na+/K+-ATPase) in the basolateral membrane of the renal epithelial cells. The reabsorption of sodium ions creates an osmotic gradient that allows water to follow passively, through the aquaporin-1 water channels in the luminal membrane of the renal epithelial cells. This process is also influenced by the presence of ADH (antidiuretic hormone), which increases the expression of aquaporin-2 water channels in the collecting ducts, facilitating further water reabsorption.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the primary mechanism of water reabsorption in the PCT is not through the reabsorption of glucose or amino acids. While glucose and amino acids are reabsorbed in the PCT, their reabsorption does not directly contribute to water reabsorption.
* **Option B:** This option is incorrect because the countercurrent multiplier system, which is involved in the concentration of the urine in the collecting ducts, is not the primary mechanism of water reabsorption in the PCT.
* **Option C:** This option is incorrect because the reabsorption of calcium ions is not directly related to the reabsorption of water in the PCT.

**Clinical Pearl / High-Yield Fact**
The PCT reabsorbs approximately 65-70% of the filtered sodium ions and 65-70% of the filtered water, making it the primary site of electrolyte and water reabsorption in the kidney. This process is critical for maintaining the body's fluid balance and electrolyte homeostasis.

**Correct Answer:** A. Sodium-potassium pump (Na+/K+-ATPase) and aquaporin-1 water channels.