**Core Concept**
Bartter syndrome is a rare genetic disorder characterized by a defect in the functioning of the ion transporters in the thick ascending limb of the loop of Henle in the kidneys. This leads to an inability to reabsorb sodium, potassium, and chloride ions, resulting in their excessive loss in the urine.

**Why the Correct Answer is Right**
Bartter syndrome is caused by mutations in genes that encode for proteins involved in ion transport, such as the Na-K-2Cl cotransporter (NKCC2) and the K+ channel ROMK. The loss of these ions leads to secondary hyperaldosteronism, which causes hypertension despite hypokalemia. The excessive loss of potassium ions in the urine is a hallmark of the condition. Additionally, patients with Bartter syndrome may exhibit metabolic alkalosis due to the loss of hydrogen ions in the urine.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hypokalemia is a feature of Bartter syndrome, making this option incorrect.
* **Option B:** Metabolic alkalosis is also a feature of Bartter syndrome, making this option incorrect.
* **Option C:** Secondary hyperaldosteronism is a characteristic of Bartter syndrome, leading to hypertension, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
Bartter syndrome is a rare genetic disorder that presents with a characteristic triad of hypokalemia, metabolic alkalosis, and hypotension, despite secondary hyperaldosteronism leading to hypertension.

**Correct Answer: A. Hypokalemia is a hallmark feature of Bartter syndrome.**