**Core Concept**
The distal convoluted tubule (DCT) is a segment of the nephron in the kidney, responsible for fine-tuning electrolyte and water balance. It plays a crucial role in the regulation of calcium, magnesium, and potassium levels in the blood.

**Why the Correct Answer is Right**
The distal convoluted tubule is approximately 5 mm in length, which is significantly shorter than the proximal convoluted tubule. This shorter length allows for a more focused and efficient reabsorption of ions and water, as well as the secretion of excess ions and waste products. The DCT contains specialized transport mechanisms, such as the thiazide-sensitive sodium-chloride cotransporter, which helps regulate sodium and chloride levels in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** 2 mm is too short for the DCT, which would compromise its ability to regulate electrolyte balance.
**Option C:** 8 mm is longer than the actual length of the DCT, which might lead to excessive reabsorption or secretion of ions.
**Option D:** 12 mm is significantly longer than the DCT, which would disrupt the normal nephron function and lead to electrolyte imbalances.

**Clinical Pearl / High-Yield Fact**
The distal convoluted tubule is also the site of action of thiazide diuretics, which inhibit the sodium-chloride cotransporter to increase urine production and reduce sodium reabsorption.

**✓ Correct Answer: B. 5 mm**