**Core Concept**
Cystic fibrosis is a genetic disorder caused by mutations in the *CFTR* gene, which encodes the cystic fibrosis transmembrane conductance regulator (CFTR) protein. This protein functions as a chloride channel in epithelial cells, and its dysfunction leads to impaired chloride and bicarbonate transport, resulting in thick, sticky mucus buildup in the lungs and other organs.

**Why the Correct Answer is Right**
In cystic fibrosis, mutations in the CFTR gene result in defective or absent chloride channel function in epithelial cells. This leads to reduced chloride secretion and increased sodium reabsorption, causing dehydration of airway surface liquid. The abnormal chloride transport disrupts ion balance and mucus viscosity, leading to chronic respiratory infections and pancreatic insufficiency. The disease is directly linked to epithelial chloride channel dysfunction.

**Why Each Wrong Option is Incorrect**
Option A: Ehlers-Danlos syndrome involves defects in collagen synthesis and connective tissue integrity, not chloride channel function.
Option B: Marfan syndrome results from mutations in the *FBN1* gene affecting fibrillin-1, leading to skeletal and cardiovascular abnormalities.
Option D: Diabetes insipidus is due to dysfunction of antidiuretic hormone (ADH) or its receptors, not epithelial chloride channels.

**Clinical Pearl / High-Yield Fact**
Remember: CFTR is the chloride channel protein mutated in cystic fibrosis. If a patient has thick mucus, recurrent lung infections, and pancreatic insufficiency, think CF — especially with a family history.

✓ Correct Answer: C. Cystic fibrosis