## **Core Concept**
Cystic fibrosis (CF) is a genetic disorder caused by mutations in a specific gene that codes for a protein involved in the transport of chloride ions across cell membranes. This protein is crucial for regulating the amount of water in sweat, digestive fluids, and mucus. The disorder leads to severe damage to the lungs, digestive system, and other organs.

## **Why the Correct Answer is Right**
The correct answer involves the gene that encodes for the **cystic fibrosis transmembrane conductance regulator (CFTR) protein**. The CFTR protein functions as a channel transporting chloride ions across epithelial cell membranes. Mutations in the CFTR gene lead to abnormal chloride transport, resulting in the production of thick, sticky mucus that clogs the airways and traps bacteria, leading to recurrent respiratory infections and lung damage. The most common mutation is a deletion of three nucleotides that results in the loss of the amino acid phenylalanine at position 508 (ΔF508) in the CFTR protein.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Incorrect because while there are numerous genes involved in various diseases, the specific mutation causing cystic fibrosis is well identified.
- **Option B:** Incorrect as it does not accurately represent the gene or protein associated with cystic fibrosis.
- **Option D:** Incorrect because it is not related to the causation of cystic fibrosis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that cystic fibrosis is caused by mutations in the **CFTR gene**, with the ΔF508 mutation being the most common cause, responsible for about 70% of CF cases worldwide. This mutation leads to misfolding of the CFTR protein, preventing it from reaching the cell surface and functioning properly.

## **Correct Answer:** . CFTR gene.