**Core Concept**
Nephrogenic diabetes insipidus (NDI) is a condition characterized by the kidneys' inability to concentrate urine in response to antidiuretic hormone (ADH). This is due to a defect in the renal collecting ducts' ability to respond to ADH, leading to excessive thirst and polyuria.

**Why the Correct Answer is Right**
The autosomal variant of NDI is caused by mutations in the aquaporin-2 (AQP2) gene, which encodes for a water channel protein expressed in the collecting ducts of the kidneys. AQP2 is responsible for water reabsorption in response to ADH, and mutations in this gene lead to impaired water reabsorption, resulting in polyuria and polydipsia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the X-linked variant of NDI is caused by mutations in the V2 vasopressin receptor gene, not the aquaporin-2 gene.
**Option B:** This option is incorrect because the autosomal dominant variant of NDI is caused by mutations in the V2 vasopressin receptor gene, not the aquaporin-2 gene.
**Option C:** This option is incorrect because the autosomal recessive variant of NDI is not a well-defined clinical entity, and the primary cause is still unknown.
**Option D:** This option is incorrect because the V2 vasopressin receptor gene is involved in the X-linked variant of NDI, not the autosomal variant.

**Clinical Pearl / High-Yield Fact**
AQP2 mutations are the primary cause of autosomal NDI, and genetic testing can help confirm the diagnosis. It's essential to distinguish between X-linked and autosomal variants of NDI, as they have different inheritance patterns and genetic causes.

**Correct Answer: A. Aquaporin-2 gene**