**Core Concept**
Barter's Syndrome is a rare genetic disorder characterized by a defect in the kidney's ability to regulate electrolyte balance. It involves a mutation in the gene encoding the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle. This leads to an imbalance in electrolyte reabsorption and secretion.

**Why the Correct Answer is Right**
The defect in the Na-K-2Cl cotransporter results in an excessive loss of potassium ions (K+) in the urine, leading to hypokalemia. This is because the mutated transporter is unable to properly reabsorb potassium ions, causing them to be excreted in the urine. Additionally, the defect also leads to an excessive reabsorption of sodium ions (Na+), which contributes to the development of hypokalemia. The loss of potassium ions is a key feature of Barter's Syndrome, and hypokalemia is a characteristic finding in affected individuals.

**Why Each Wrong Option is Incorrect**
**Option A:** Metabolic acidosis is not typically seen in Barter's Syndrome. The disorder is more commonly associated with metabolic alkalosis due to the loss of potassium ions and the excessive reabsorption of sodium ions.
**Option C:** Hyperkalemia is the opposite of what is seen in Barter's Syndrome. The excessive loss of potassium ions in the urine leads to hypokalemia, not hyperkalemia.
**Option D:** The disorder is actually associated with an increase in urinary calcium, not a decrease. The excessive reabsorption of sodium ions and the loss of potassium ions lead to an increase in the reabsorption of calcium ions, resulting in an increase in urinary calcium excretion.

**Clinical Pearl / High-Yield Fact**
Barter's Syndrome is a classic example of a disorder that presents with hypokalemia and metabolic alkalosis. It is essential to consider this diagnosis in patients with unexplained hypokalemia and metabolic alkalosis, particularly in the context of a family history of the disorder.

**✓ Correct Answer: B. Hypokalemia**