**Core Concept**
Bronchiectasis is a chronic respiratory disease characterized by irreversible dilatation of bronchi due to damage to the bronchial wall, leading to impaired clearance of mucus and recurrent infections. High-resolution computed tomography (HRCT) scan is the preferred imaging modality for diagnosing bronchiectasis due to its ability to provide detailed images of the airways.

**Why the Correct Answer is Right**
HRCT scan is the imaging modality of choice for bronchiectasis due to its high spatial resolution, which allows for the detection of subtle changes in the bronchial wall and airway dilatation. It can also provide information on the extent and severity of bronchiectasis, including the number of affected bronchi and the presence of complications such as bronchial stenosis or bronchiolitis. The detailed images obtained from HRCT scans enable accurate assessment of bronchiectasis, which is essential for planning treatment and monitoring disease progression.

**Why Each Wrong Option is Incorrect**
**Option B:** Spiral CT, also known as helical CT, provides high-speed scanning and is useful for detecting lung nodules and masses. However, it is not as sensitive as HRCT for detecting bronchiectasis due to its lower spatial resolution.

**Option C:** Bronchoscopy is a valuable tool for diagnosing bronchiectasis, particularly for detecting complications such as bronchial stenosis or bronchiectasis-related bleeding. However, it is not the initial imaging modality of choice for diagnosing bronchiectasis.

**Option D:** Pulmonary angiography is a diagnostic imaging modality used to visualize the pulmonary vasculature and diagnose pulmonary embolism. It is not relevant for diagnosing bronchiectasis.

**Clinical Pearl / High-Yield Fact**
HRCT scans should be performed with thin slices (1-2 mm) and reconstructed images in both the axial and coronal planes to maximize diagnostic accuracy. This allows for detailed assessment of the bronchial wall and airway dilatation, which is essential for diagnosing bronchiectasis.

**✓ Correct Answer: A. HRCT scan**