**Core Concept**
Bronchiectasis is a condition characterized by permanent dilation of parts of the airways due to chronic inflammation and damage. Diagnosing bronchiectasis can be challenging, and a gold standard is needed to confirm its presence.

**Why the Correct Answer is Right**
High-resolution computed tomography (HRCT) is considered the gold standard for diagnosing bronchiectasis, particularly in cases of minimal bronchiectasis. HRCT scans provide detailed images of the lung parenchyma and airways, allowing for the detection of subtle changes associated with bronchiectasis, such as bronchial wall thickening, luminal dilatation, and mucoid impaction. The scans are typically obtained at full inspiration to ensure accurate assessment of the airway dimensions.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not sensitive enough to detect minimal bronchiectasis, as it primarily shows large airway changes and may not capture the subtle changes associated with bronchiectasis.

**Option B:** Sputum cytology and culture can provide information on the presence of infectious agents and inflammation, but they do not directly assess the structural changes characteristic of bronchiectasis.

**Option C:** Pulmonary function tests (PFTs) may show abnormalities in lung function, such as obstructive or restrictive patterns, but they do not provide direct evidence of bronchiectasis.

**Clinical Pearl / High-Yield Fact**
When interpreting HRCT scans for bronchiectasis, it's essential to look for the presence of "tram-track" or "railroad-track" signs, which refer to the alternating areas of bronchial dilatation and airway collapse, indicative of bronchiectasis.

**Correct Answer: C. High-resolution computed tomography (HRCT) scan.**