**Core Concept**
Cystic fibrosis is a genetic disorder caused by mutations in the CFTR gene, leading to defective chloride and sodium transport in epithelial cells. This results in thick, sticky mucus in lungs and digestive tracts, with a hallmark diagnostic feature being abnormally high chloride levels in sweat.

**Why the Correct Answer is Right**
In cystic fibrosis, the defective CFTR protein impairs chloride channel function, causing reduced chloride reabsorption in sweat glands. This leads to increased chloride secretion into sweat. Sweat chloride levels are typically >60 mmol/L in affected individuals, making elevated sweat chloride the gold-standard diagnostic marker. This test is highly specific and used routinely in pediatric screening.

**Why Each Wrong Option is Incorrect**
Option A: Low sweat chloride levels are seen in conditions like hypothyroidism or renal tubular acidosis, not cystic fibrosis.
Option C: Sweat sodium levels are actually elevated in CF due to impaired sodium reabsorption, not low.
Option D: Potassium levels in sweat are usually normal or slightly elevated, not significantly elevated in CF.

**Clinical Pearl / High-Yield Fact**
A sweat chloride test >60 mmol/L is diagnostic of cystic fibrosis; it is the most reliable non-invasive test in pediatric patients with suspected CF. Always perform this test in infants with chronic respiratory symptoms or family history of CF.

✓ Correct Answer: B. Elevated sweat chloride levels