**Core Concept**
Bronchiectasis is characterized by irreversible dilation of bronchial airways due to chronic inflammation and damage. The investigation of choice must accurately detect and characterize the extent of airway wall thickening, tree-in-bud appearance, and volume enlargement, which are best visualized using high-resolution computed tomography (HRCT) of the lungs.

**Why the Correct Answer is Right**
HRCT lung provides superior spatial resolution compared to other imaging modalities, enabling detection of subtle airway wall thickening, mucus plugging, and tree-in-bud patterns—hallmark features of bronchiectasis. It is highly sensitive and specific, allowing for early diagnosis, assessment of disease severity, and monitoring of treatment response. Unlike chest X-ray, HRCT can identify early changes before they become visible on X-ray, and unlike MRI or bronchoscopy, it is non-invasive and widely accessible.

**Why Each Wrong Option is Incorrect**
Option A: Bronchoscopy is invasive and primarily used for diagnostic procedures like biopsy or brushings, not for initial screening or evaluation of bronchiectasis. It is not the investigation of choice due to its limited role in imaging and diagnostic workup.
Option B: MRI thorax has poor soft-tissue contrast and limited availability for routine lung evaluation. It is not recommended for detecting bronchial dilation or airway abnormalities.
Option D: Chest X-ray is insensitive in early or mild bronchiectasis and often misses subtle changes. It may show normal findings even in advanced disease, leading to underdiagnosis.

**Clinical Pearl / High-Yield Fact**
HRCT is the gold standard for diagnosing bronchiectasis—its findings are essential for differentiating bronchiectasis from other lung conditions like COPD or asthma. A "tree-in-bud" pattern on HRCT is pathognomonic for bronchiectasis and should be recognized in clinical practice.

✓ Correct Answer: C. HRCT lung