**Core Concept**
Cystic fibrosis is a genetic disorder characterized by the production of thick, sticky mucus due to mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene. This mutation leads to impaired chloride transport across epithelial membranes, resulting in abnormal sweat gland function and respiratory complications.

**Why the Correct Answer is Right**
The sweat chloride test is a diagnostic tool for cystic fibrosis, as it measures the concentration of chloride ions in sweat. In individuals with cystic fibrosis, the abnormal CFTR protein leads to increased chloride reabsorption in the sweat glands, resulting in elevated sweat chloride levels (typically >60 mmol/L). This test is widely used as a screening tool for cystic fibrosis, particularly in newborns and infants.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood glucose levels are not directly related to the diagnosis of cystic fibrosis.
**Option B:** A chest X-ray may show pulmonary complications of cystic fibrosis, but it is not diagnostic of the condition.
**Option C:** Liver function tests may be abnormal in individuals with cystic fibrosis due to complications such as bile duct obstruction, but they are not specific for the diagnosis of cystic fibrosis.

**Clinical Pearl / High-Yield Fact**
The sweat chloride test is the primary diagnostic tool for cystic fibrosis, but it is essential to interpret results in conjunction with clinical symptoms and family history.

**Correct Answer: C. Sweat chloride test.**