**Core Concept**
Bartter's Syndrome is a rare genetic disorder characterized by a defect in the function of the loop of Henle in the kidneys. This defect leads to a disturbance in electrolyte and acid-base balance, resulting in various clinical manifestations.

**Why the Correct Answer is Right**
The correct answer is D. Decrease in urinary calcium. In Bartter's Syndrome, the kidneys are unable to reabsorb calcium properly, leading to an increase in urinary calcium excretion. This is due to the impaired function of the thick ascending limb of the loop of Henle, which is responsible for calcium reabsorption. As a result, patients with Bartter's Syndrome typically have **hypercalciuria**, not hypocalciuria.

**Why Each Wrong Option is Incorrect**
**Option A:** Metabolic alkalosis is a common finding in Bartter's Syndrome due to the impaired potassium and hydrogen ion reabsorption in the kidneys. The elevated bicarbonate levels in the blood lead to metabolic alkalosis.

**Option B:** Hypokalemia is a hallmark of Bartter's Syndrome. The impaired potassium reabsorption in the kidneys leads to excessive potassium loss in the urine, resulting in low potassium levels in the blood.

**Option C:** Hypomagnesemia is also seen in Bartter's Syndrome. The impaired magnesium reabsorption in the kidneys leads to excessive magnesium loss in the urine, resulting in low magnesium levels in the blood.

**Clinical Pearl / High-Yield Fact**
Bartter's Syndrome is a classic example of a renal tubular acidosis (RTA) that presents with hypokalemia, metabolic alkalosis, and hypercalciuria. It is essential to consider this diagnosis in patients with these clinical features, especially in the context of a family history or genetic predisposition.

**✓ Correct Answer: D. Decrease in urinary calcium**