A 6 year old boy presented with recurrent chest infections. Sweat chloride test was done and it shows raised values. What is the next BEST investigation to confirm the diagnosis?
**Core Concept**
The patient's symptoms and elevated sweat chloride levels are suggestive of Cystic Fibrosis (CF), a genetic disorder affecting chloride ion transport across epithelial membranes. This abnormal transport leads to thick and sticky mucus production, causing recurrent respiratory infections and other systemic complications.
**Why the Correct Answer is Right**
The next best investigation to confirm the diagnosis of CF is a genetic test, specifically a **DNA analysis for the CFTR gene mutation**. This test can identify the underlying genetic defect responsible for the abnormal chloride transport, confirming the diagnosis of CF. The CFTR gene encodes for the cystic fibrosis transmembrane conductance regulator protein, which is essential for chloride ion transport across epithelial membranes.
**Why Each Wrong Option is Incorrect**
**Option A:** A chest X-ray may show signs of chronic lung disease, but it is not specific for CF and cannot confirm the diagnosis.
**Option B:** A complete blood count (CBC) may show signs of infection or inflammation, but it is not relevant to the diagnosis of CF.
**Option C:** A pulmonary function test (PFT) may show abnormal lung function, but it is not specific for CF and cannot confirm the diagnosis.
**Clinical Pearl / High-Yield Fact**
CF is caused by mutations in the CFTR gene, which codes for a chloride channel. The most common mutation is the delta F508 mutation, which leads to a truncated and non-functional protein.
**Correct Answer: D. DNA analysis for the CFTR gene mutation**