## **Core Concept**
Mucoviscidosis, also known as cystic fibrosis (CF), is a genetic disorder that primarily affects the lungs and digestive system. It is caused by mutations in the gene encoding for a specific protein, leading to abnormal mucus production. The underlying principle being tested here involves understanding the pathophysiology of cystic fibrosis, particularly the defective protein responsible for the disease.

## **Why the Correct Answer is Right**
The correct answer, , refers to the cystic fibrosis transmembrane conductance regulator (CFTR) protein. CFTR is a chloride channel that plays a crucial role in regulating the transport of salt and water in and out of cells, particularly in epithelial tissues. The most common defect in CF is a deletion mutation in the CFTR gene, leading to the production of thick, sticky mucus that clogs the airways and traps bacteria, resulting in recurrent respiratory infections and pancreatic insufficiency. The defective CFTR protein fails to function properly, disrupting chloride transport and leading to the clinical manifestations of cystic fibrosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while pancreatic enzyme insufficiency is a complication of cystic fibrosis, it is not the primary defect.
- **Option B:** This option is incorrect as it does not directly relate to the primary defect in cystic fibrosis.
- **Option C:** This option is incorrect because although abnormalities in sweat gland function are a diagnostic criterion for cystic fibrosis (as measured by sweat chloride tests), they are a consequence of the defective CFTR protein, not the primary defect itself.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **F508del mutation** is the most common mutation causing cystic fibrosis, responsible for approximately 70% of CF cases in individuals of European descent. This mutation leads to a deletion of three nucleotides in the CFTR gene, resulting in the loss of the amino acid phenylalanine at position 508 (ΔF508) of the CFTR protein. This mutation causes misfolding of the CFTR protein, preventing it from reaching the cell surface and functioning as a chloride channel.

## **Correct Answer:** . CFTR protein.