**Question:** Which of the following is NOT true for Cystic fibrosis: March 2013

**Core Concept:** Cystic Fibrosis (CF) is a genetic disorder caused by mutations in the CFTR gene, which encodes for the cystic fibrosis transmembrane conductance regulator protein. The protein plays a crucial role in regulating the movement of chloride ions and water across cell membranes, particularly in the exocrine glands. Mutations in the CFTR gene lead to the production of an abnormal protein, resulting in thick, sticky mucus that obstructs and damages the lungs, pancreas, liver, and other organs.

**Why the Correct Answer is Right:** Cystic fibrosis is a progressive genetic disorder with a wide range of symptoms and complications. The correct answer, "A. Mucus viscosity is decreased in CF," is not true because the mutations in the CFTR gene cause the production of an abnormal protein leading to increased mucus viscosity, making it thick and sticky. This thick mucus leads to the obstruction and damage of organs like lungs and pancreas.

**Why Each Wrong Option is Incorrect:**

B. Mucus viscosity is increased in the pancreas only: While mucus viscosity is indeed increased in the pancreas, cystic fibrosis affects multiple organ systems. Mucus viscosity is increased not only in the pancreas but also in the lungs, liver, and other organs.

C. CF is caused by a deficiency in chloride ions: The core concept is that cystic fibrosis is caused by mutations in the CFTR gene resulting in abnormal CFTR protein production, not a deficiency in chloride ions.

D. CFTR gene mutations are rare: While some CFTR gene mutations are rare, the vast majority of CF cases are caused by well-known mutations, such as F508del, G551D, and R117H.

**Clinical Pearl:** Cystic fibrosis is a genetic disorder with a significant impact on multiple organ systems. Understanding the underlying molecular basis (CFTR gene mutations) and the clinical manifestations (increased mucus viscosity) are crucial for diagnosing and managing patients with cystic fibrosis.