**Core Concept:** Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a genetic disorder caused by mutations in the PKD1 or PKD2 genes, which encode polycystin-1 and polycystin-2 proteins, respectively. These proteins play a role in cell signaling pathways and development of the kidneys and other organs. The disease is characterized by the formation of numerous fluid-filled cysts in the kidneys, leading to renal failure over time.

**Why the Correct Answer is Right:** Chromosome 16 is associated with autosomal dominant polycystic kidney disease (ADPKD). The PKD1 gene, located on chromosome 16p13.3, encodes the polycystin-1 protein, which is involved in cell signaling pathways and kidney development. Mutations in this gene cause ADPKD. The PKD1 gene is responsible for approximately 85% of cases with ADPKD.

**Why Each Wrong Option is Incorrect:**

A. Chromosome 4 is incorrect because the PKD1 gene responsible for ADPKD is located on chromosome 16, not 4.

B. Chromosome 17 is incorrect as the PKD2 gene associated with ADPKD is located on chromosome 4, not 17. PKD2 mutations cause a rare form of ADPKD called TRIPKD (T hereditary renal autosomal dominant polycystic disease with liver lesions).

C. Chromosome 20 is incorrect as it does not contain any PKD genes associated with ADPKD. The PKD1 gene is located on chromosome 16, while PKD2 is located on chromosome 4.

D. Chromosome X is incorrect as ADPKD is an autosomal dominant disorder, meaning it is inherited through both parents to their offspring. Therefore, the disease cannot be associated with X-linked inheritance.

**Clinical Pearl:** Polycystic kidney disease (PKD) can be distinguished from other kidney diseases by assessing the inheritance pattern and the number of cysts in the kidneys and liver. ADPKD is characterized by the presence of multiple cysts in the kidneys and liver, while PKD2 is inherited X-linked and PKD1/PKD2 mutations can cause liver cysts as well as pancreatic and liver cysts. Autosomal dominant polycystic liver disease (ADPLD) is caused by mutations in PKHD1 gene and is characterized by liver cysts and biliary cirrhosis but no renal cysts.

**Correct Answer:** Chromosome 16p13.3 (PKD1 gene)

In conclusion, ADPKD is caused by mutations in the PKD1 gene located on chromosome 16p13.3, which encodes polycystin-1 protein involved in cell signaling pathways and kidney development. This gene is responsible for approximately 85% of ADPKD cases.