**Core Concept**
Water reabsorption in the renal tubules is a crucial process that helps maintain fluid balance in the body. The majority of water reabsorption occurs in the proximal convoluted tubule and the descending limb of the loop of Henle. Water reabsorption is tightly coupled with sodium reabsorption, a process known as solute-linked water reabsorption.

**Why the Correct Answer is Right**
The bulk of water reabsorption occurs secondary to sodium reabsorption due to the osmotic gradient generated by the reabsorption of sodium ions. In the proximal convoluted tubule, sodium ions are reabsorbed into the peritubular capillaries, creating an osmotic gradient that draws water into the interstitium through the process of osmosis. This process is mediated by the sodium-potassium pump, which maintains the high sodium concentration in the tubular fluid. As a result, water follows the sodium ions and is reabsorbed into the bloodstream. This mechanism ensures that the majority of water reabsorption occurs in conjunction with sodium reabsorption.

**Why Each Wrong Option is Incorrect**
**Option B:** Majority of facultative reabsorption occurs in proximal tubule
Facultative reabsorption, also known as regulated reabsorption, occurs in the collecting duct and is dependent on the presence of antidiuretic hormone (ADH). While the proximal tubule does reabsorb a significant amount of water, facultative reabsorption is not the primary mechanism in this segment.

**Option C:** Obligatory reabsorption is ADH dependent
Obligatory reabsorption, also known as fixed reabsorption, occurs in the proximal convoluted tubule and is not dependent on ADH. This process is driven by the osmotic gradient generated by sodium reabsorption, as described earlier.

**Option D:** 20% of water is always reabsorbed irrespective of water balance
While it is true that a significant amount of water is reabsorbed in the proximal tubule, the exact percentage can vary depending on the body's fluid balance. In states of dehydration, more water may be reabsorbed to conserve fluid, while in states of excess fluid, less water may be reabsorbed.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that water reabsorption is tightly coupled with sodium reabsorption, and any disorder affecting sodium reabsorption can impact water reabsorption. For example, a patient with a sodium-wasting disorder, such as nephrogenic diabetes insipidus, may experience excessive water loss due to impaired sodium reabsorption.

**✓ Correct Answer: A. The bulk of water reabsorption occurs secondary to Na+ reabsorption**