**Question:** In a child with respiratory distress, failure to thrive. His sweat chloride levels were estimated at 35 mEq/L and 41 mEq/L. What is the next best test to diagnose cystic fibrosis?

**Core Concept:** Cystic Fibrosis (CF) is a genetic disorder that primarily affects the respiratory and gastrointestinal systems. It is caused by mutations in the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene, leading to the production of thick, sticky mucus that blocks airways and causes lung infections, and hinders the digestive system, leading to malabsorption and poor growth. Sweat chloride levels are elevated in CF patients, reflecting chloride ion imbalance in sweat gland ducts.

**Why the Correct Answer is Right:** To diagnose CF, we need to evaluate the sweat chloride levels further. Given the values of 35 mEq/L and 41 mEq/L, the most appropriate test to perform next is a sweat chloride test. This test measures the sweat chloride concentration to confirm CF diagnosis as it is highly specific and sensitive for CF.

**Why Each Wrong Option is Incorrect:**

A. Blood tests (e.g., CBC, electrolytes) are essential in assessing general health and detecting secondary complications but do not directly diagnose CF.

B. Genetic testing for CFTR gene mutations is crucial for confirming CF inheritance but does not directly provide a definitive diagnosis.

C. Imaging studies (X-ray, CT scan) help detect lung abnormalities but do not directly diagnose CF.

D. Clinical evaluation and growth assessment are important but do not directly diagnose CF.

**Clinical Pearl:** A positive sweat chloride test with elevated levels (>60 mEq/L) strongly suggests CF, while levels between 30-60 mEq/L warrant further evaluation, like a molecular genetic testing for CFTR mutations, to confirm the diagnosis. If sweat chloride levels are normal (<30 mEq/L), the clinical suspicion of CF is low, and further diagnostic testing is not necessary. **Correct Answer:** B. Genetic testing for CFTR gene mutations. **Explanation:** Genetic testing for CFTR gene mutations is crucial to confirm the inheritance pattern of CF, thus helping in identifying carriers and family planning. Although genetic testing alone cannot diagnose CF, it complements the sweat chloride test results to confirm the diagnosis.