## **Core Concept**
The child's symptoms suggest a multisystem disorder involving chronic respiratory infections, gastrointestinal malabsorption, and possibly an underlying immune deficiency or cystic fibrosis. The presence of nasal polyps in a child is highly suggestive of cystic fibrosis (CF), a genetic disorder that affects multiple organ systems.

## **Why the Correct Answer is Right**
Cystic fibrosis is caused by mutations in the **CFTR (cystic fibrosis transmembrane conductance regulator) gene**, leading to abnormal chloride transport across epithelial membranes. This results in thick, viscous secretions that clog the airways and trap bacteria, leading to recurrent respiratory infections, and also affects the pancreas, impairing digestive enzyme release and causing malabsorption. The **sweat chloride test** is the gold standard diagnostic test for CF. It measures the chloride concentration in sweat; patients with CF have elevated levels (>60 mmol/L) due to the abnormal CFTR protein.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a chest X-ray might show signs suggestive of cystic fibrosis, such as bronchiectasis, it is not diagnostic on its own.
- **Option B:** A nasal smear for eosinophils might be helpful in diagnosing allergic rhinitis but is not specific for cystic fibrosis.
- **Option C:** A pancreatic enzyme test might indicate pancreatic insufficiency but is not the primary diagnostic test for cystic fibrosis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **nasal polyps in children are highly suggestive of cystic fibrosis** until proven otherwise. Early diagnosis of CF is crucial for management and prevention of complications. The sweat chloride test, though simple, requires careful execution to avoid false results.

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