**Core Concept**
Salt-losing nephropathy, also known as medullary cystic kidney disease, is a rare genetic disorder affecting the kidneys, leading to progressive renal failure. This condition is characterized by the loss of sodium and chloride ions in the urine, resulting in dehydration and electrolyte imbalance.

**Why the Correct Answer is Right**
The primary defect in salt-losing nephropathy involves the thick ascending limb of the loop of Henle, where the Na-K-2Cl cotransporter (NKCC2) is impaired. This leads to an inability to reabsorb sodium, potassium, and chloride ions, resulting in their excessive excretion in the urine. The kidneys' inability to concentrate urine further exacerbates the condition, causing polyuria, polydipsia, and electrolyte imbalances.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not accurately describe the primary defect in salt-losing nephropathy.

**Option B:** This option is incorrect as it does not specify the correct location of the defect in the kidneys.

**Option C:** This option is incorrect as it does not mention the Na-K-2Cl cotransporter (NKCC2), which is crucial in the pathophysiology of salt-losing nephropathy.

**Clinical Pearl / High-Yield Fact**
A key feature of salt-losing nephropathy is the excessive excretion of sodium, potassium, and chloride ions in the urine, leading to dehydration and electrolyte imbalances. Early recognition and management of this condition are essential to prevent further renal damage.

**Correct Answer: C.** Medullary cystic kidney disease with the defect in the Na-K-2Cl cotransporter (NKCC2) in the thick ascending limb of the loop of Henle.