**Question:** Autosomal recessive polycystic kidney disease is characterised by the altered expression of:

A. PKD1
B. PKD2
C. PKD1L1
D. PKD3

**Correct Answer:** A. PKD1

**Core Concept:**
Autosomal Recessive Polycystic Kidney Disease (ARPKD) is a rare genetic disorder characterized by the abnormal development and growth of cysts in the kidneys, liver, and other organs. In this case, we are discussing the altered expression of genes responsible for the disease.

**Why the Correct Answer is Right:**
ARPKD is caused by mutations in one of two genes: PKD1 (located on chromosome 16) or PKD2 (located on chromosome 4). Out of these, PKD1 is the more common cause of ARPKD. The altered expression of PKD1, located on chromosome 16, results in the formation of cysts in various organs, leading to the characteristic symptoms of the disease.

**Why Each Wrong Option is Incorrect:**
B. PKD2 (chromosome 4) is the less common cause of ARPKD. Mutations in PKD2 do not explain the disease, making option B incorrect.
C. PKD1L1 is a closely related gene to PKD1 but is not directly involved in causing ARPKD. The presence of PKD1L1 mutations does not result in the formation of cysts in the kidneys or other organs, making option C incorrect.
D. PKD3 is another gene related to PKD1, but it is not responsible for ARPKD. Mutations in PKD3 do not lead to the development of cysts in the kidneys or other organs, making option D incorrect.

**Clinical Pearl:**
Understanding the genetic basis of ARPKD and its associated genes is crucial for diagnosing and managing patients with this condition. Knowledge of PKD1 and PKD2 is essential for clinicians dealing with patients presenting with symptoms of the disease.