## **Core Concept**
Cystic fibrosis (CF) is a genetic disorder caused by mutations in the **CFTR (cystic fibrosis transmembrane conductance regulator) gene**, which encodes for a chloride channel. This mutation leads to abnormal chloride transport across epithelial membranes, affecting multiple organ systems. One of the diagnostic hallmarks of CF is an elevated sweat chloride level.

## **Why the Correct Answer is Right**
The correct answer, **> 60 mEq/L**, is right because in cystic fibrosis, the defective CFTR protein leads to an inability to reabsorb chloride ions in the sweat glands. As a result, individuals with CF have higher concentrations of chloride in their sweat. A sweat chloride test is used for the diagnosis of CF, with values > 60 mEq/L considered diagnostic for CF in the appropriate clinical context.

## **Why Each Wrong Option is Incorrect**
- **Option A: < 30 mEq/L** - This is incorrect because sweat chloride levels in individuals without cystic fibrosis are typically 120 mEq/L** - While sweat chloride levels in CF patients are indeed elevated, stating "> 120 mEq/L" is too specific and does not accurately represent the diagnostic threshold.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the sweat test should be performed in a specialized laboratory with quality control measures in place, and the result should be interpreted in the context of the patient's clinical presentation and family history. A diagnosis of CF requires a comprehensive evaluation, including clinical findings, family history, and sweat chloride test results.

## **Correct Answer: B. > 60 mEq/L**