**Core Concept**
Cystic fibrosis (CF) is a genetic disorder caused by mutations in the CFTR (cystic fibrosis transmembrane conductance regulator) gene, leading to abnormal chloride ion transport and subsequent thickened mucus production. This results in recurrent respiratory infections, pancreatic insufficiency, and other complications.

**Why the Correct Answer is Right**
To confirm the diagnosis of cystic fibrosis, the next step is to perform a genetic test to identify the specific mutations in the CFTR gene. This can be done through DNA sequencing or PCR (polymerase chain reaction) to detect the most common mutations associated with CF. The sweat chloride test, though useful, is not definitive and can have false positives or negatives.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chest X-ray is a useful tool for assessing lung involvement in CF but does not confirm the diagnosis. It can show evidence of bronchiectasis, pneumothorax, or other complications but does not identify the underlying genetic defect.
* **Option B:** Complete blood count (CBC) may show signs of chronic infection or inflammation but is not specific for CF. It can be normal in early stages or in asymptomatic carriers.
* **Option C:** ECG (electrocardiogram) may show signs of right ventricular hypertrophy or other cardiac complications associated with CF but is not a diagnostic test for the condition.

**Clinical Pearl / High-Yield Fact**
Cystic fibrosis is a multisystem disorder with a wide range of clinical manifestations, but the presence of recurrent respiratory infections and exocrine pancreatic insufficiency in a young child should raise suspicion for CF. Early diagnosis and treatment can significantly improve outcomes.

**Correct Answer:** D. Genetic testing to identify CFTR gene mutations.