**Core Concept**
Adult polycystic kidney disease (APKD) is a genetic disorder characterized by the growth of numerous cysts filled with fluid in the kidneys, leading to kidney enlargement and impaired kidney function. This condition is caused by mutations in the PKD1 or PKD2 genes, which encode for polycystin-1 and polycystin-2 proteins, respectively. These proteins play a crucial role in maintaining the normal structure and function of the kidney.

**Why the Correct Answer is Right**
The correct answer is related to the inheritance pattern of APKD. APKD is an autosomal dominant disorder, meaning that a single copy of the mutated gene is sufficient to cause the disease. This means that if one parent has the disease, each child has a 50% chance of inheriting the mutated gene. The polycystin-1 and polycystin-2 proteins are involved in the regulation of intracellular calcium levels and cell-cell interactions, which are critical for maintaining normal kidney function.

**Why Each Wrong Option is Incorrect**
**Option A:** APKD is primarily caused by mutations in the PKD1 gene. This is correct, as the PKD1 gene is responsible for approximately 85% of APKD cases.

**Option B:** APKD is characterized by the growth of numerous cysts in the kidneys. This is correct, as the disease is defined by the development of multiple cysts filled with fluid in the kidneys.

**Option C:** APKD is associated with an increased risk of developing intracranial aneurysms and hemorrhage. This is correct, as patients with APKD have a higher risk of developing intracranial aneurysms and hemorrhage due to the abnormal structure and function of blood vessels in the brain.

**Option D:** APKD is a recessive disorder, requiring two copies of the mutated gene to express the disease. This is incorrect, as APKD is an autosomal dominant disorder, requiring only one copy of the mutated gene to cause the disease.

**Clinical Pearl / High-Yield Fact**
APKD is often asymptomatic in the early stages, but as the disease progresses, patients may experience symptoms such as flank pain, hematuria, and hypertension. Regular screening for APKD is essential in families with a history of the disease.

**Correct Answer:** D.