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

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

**Core Concept:** Autosomal Recessive Polycystic Kidney Disease (ARPKD) is a genetic disorder caused by mutations in one of two genes: PKD1 (located on chromosome 16) and PKD2 (located on chromosome 4). This disease is inherited in an autosomal recessive manner, which means both parents must be carriers of the mutated gene to have a child affected by the disease.

**Why the Correct Answer is Right:** ARPKD is mainly caused by mutations in PKD1 gene. The PKD1 gene encodes polycystin-1 protein, which plays a crucial role in maintaining the integrity of the renal tubules and the liver bile ducts. Altered expression of PKD1 leads to the formation of multiple cysts in the kidneys and liver, causing the characteristic features of ARPKD.

**Why Each Wrong Option is Incorrect:**

A. PKD2 (Option B) is not the correct answer because it is associated with autosomal dominant polycystic kidney disease (ADPKD), which has different clinical features and inheritance pattern.

B. PKD1L1 (Option C) is not the correct answer because it is a pseudogene, which is a non-functional gene related to PKD1 but does not cause the disease. PKD1L1 is a pseudogene located near PKD1 and shares similar genetic polymorphisms with PKD1.

D. PKDREJ (Option D) is not the correct answer because it is a term used for the recombinant PKD1, which is a fragment of the PKD1 gene, not the actual disease-causing gene.

**Clinical Pearl:** ARPKD is a severe disease with high morbidity and mortality rates. Patients usually present with kidney and liver cysts, hypertension, and congenital malformations. Early diagnosis and intervention are crucial for improving the patient's prognosis.

**Correct Answer:** PKD1 (Option A) is the correct answer, as it is the gene responsible for causing Autosomal Recessive Polycystic Kidney Disease (ARPKD). Mutations in PKD1 lead to the production of non-functional polycystin-1 protein, which results in cyst formation in the kidneys and liver, causing the clinical features of ARPKD.