**Core Concept**
Cystic fibrosis (CF) is a genetic disorder caused by mutations in the CFTR gene, leading to abnormal chloride transport across epithelial cells, resulting in thickened secretions and various organ dysfunctions. The sweat chloride test is a diagnostic tool used to aid in the diagnosis of CF, with elevated levels indicating the presence of the disease.

**Why the Correct Answer is Right**
The correct answer is a genetic test for the CFTR gene mutation, specifically the delta F508 mutation, which is the most common mutation associated with CF. This test will help confirm the diagnosis by identifying the underlying genetic defect. The delta F508 mutation is a frameshift mutation that leads to the premature termination of the CFTR protein, resulting in its complete absence or a dysfunctional protein.

**Why Each Wrong Option is Incorrect**
**Option A:** A chest X-ray would be useful in evaluating the extent of lung involvement but would not support the diagnosis of CF. It is not a diagnostic test for CF.
**Option B:** A pancreatic enzyme replacement therapy (PERT) trial may be used to manage symptoms of exocrine pancreatic insufficiency but is not a diagnostic test for CF.
**Option C:** A methacholine challenge test is used to diagnose bronchial hyperresponsiveness and is not directly related to the diagnosis of CF.

**Clinical Pearl / High-Yield Fact**
CF is a multisystem disorder that affects not only the lungs and pancreas but also the digestive system, reproductive system, and other organs. Patients with CF are at increased risk of developing liver disease, including liver cysts and cirrhosis.

**Correct Answer:** C. Genetic testing for the CFTR gene mutation.