**Core Concept:**
Bronchiectasis is a condition characterized by irreversible widening and distortion of the bronchi, leading to impaired lung function and recurrent respiratory infections. The diagnosis depends on demonstrating the characteristic findings on imaging studies.

**Why the Correct Answer is Right:**
The correct answer is **A**: High-resolution computed tomography (HRCT) scan of the chest. HRCT is the most sensitive and specific imaging modality for diagnosing bronchiectasis, as it can demonstrate the typical findings such as irregular bronchial walls, airway wall thickening, and airway wall irregularities. Other diagnostic tools like X-ray or sputum examination are less sensitive and specific, and plain chest X-ray may not detect early-stage bronchiectasis.

**Why Each Wrong Option is Incorrect:**

**Option B:** Chest X-ray is less sensitive and specific, making it less reliable for diagnosing bronchiectasis, especially in its early stages.

**Option C:** Sputum examination is primarily used for identifying pathogens and is not a reliable imaging modality for diagnosing bronchiectasis.

**Option D:** Bronchoscopy is a diagnostic procedure for airway disorders and is not a diagnostic imaging tool for bronchiectasis.

**Clinical Pearl:**
In clinical practice, utilizing a combination of medical history, physical examination, and appropriate imaging studies is essential for diagnosing bronchiectasis accurately and initiating appropriate management strategies.

**Correct Answer:** **A.** High-resolution computed tomography (HRCT) scan of the chest.