**Core Concept**
The distal convoluted tubule plays a crucial role in regulating electrolyte balance and acid-base homeostasis in the kidneys. The buffer system present in this segment is essential for maintaining the body's acid-base equilibrium.

**Why the Correct Answer is Right**
The most important buffer system in the distal convoluted tubule is the ammonia buffer system. This system involves the production of ammonia (NH3) from glutamine by the enzyme glutaminase. Ammonia then combines with hydrogen ions (H+) to form ammonium ions (NH4+), which are excreted in the urine. This process helps to reduce the concentration of hydrogen ions in the blood and maintain acid-base balance. The ammonia buffer system is particularly important in the distal convoluted tubule, where it helps to regulate the pH of the tubular fluid.

**Why Each Wrong Option is Incorrect**
**Option A:** The bicarbonate buffer system is primarily active in the proximal convoluted tubule, where it helps to reabsorb bicarbonate ions.
**Option B:** The phosphate buffer system is also present in the kidneys, but it is not the most important buffer system in the distal convoluted tubule.
**Option C:** The sodium-hydrogen exchanger (NHE3) is an important transport mechanism in the proximal convoluted tubule, but it is not a buffer system.

**Clinical Pearl / High-Yield Fact**
The ammonia buffer system is particularly effective in buffering acid loads in the kidneys, and its dysfunction can lead to hyperchloremic metabolic acidosis.

**Correct Answer: B. The ammonia buffer system is the most important buffer system in the distal convoluted tubule, playing a crucial role in regulating acid-base balance.**