## **Core Concept**
Cystic fibrosis (CF) is a genetic disorder that primarily affects the lungs and digestive system. It is caused by mutations in a specific gene that codes for a protein involved in the transport of chloride ions across cell membranes. The disorder is inherited in an autosomal recessive pattern.

## **Why the Correct Answer is Right**
The correct answer is **. 7**, as cystic fibrosis is caused by mutations in the **CFTR (cystic fibrosis transmembrane conductance regulator)** gene, which is located on **chromosome 7**. The CFTR protein functions as a channel transporting chloride ions across cell membranes, and its dysfunction leads to the production of thick, sticky mucus that clogs the airways and traps bacteria, resulting in recurrent respiratory infections and other complications.

## **Why Each Wrong Option is Incorrect**
- **Option A:** . **Chromosome 1** is not associated with cystic fibrosis. While chromosome 1 does carry genes for various conditions, it is not linked to CF.
- **Option B:** . **Chromosome 3** does not carry the gene responsible for cystic fibrosis. Chromosome 3 is associated with different genetic disorders.
- **Option D:** . **Chromosome X** (sex chromosome) is not the location of the CFTR gene. Cystic fibrosis is an autosomal disorder, not a sex-linked disorder.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that cystic fibrosis is caused by mutations in the **CFTR gene on chromosome 7**. A classic clinical correlation is the **sweat chloride test**, which is diagnostic for cystic fibrosis; patients with CF have elevated sweat chloride levels due to the defective CFTR protein.

## **Correct Answer:** . 7