**Core Concept**
The patient's symptoms of recurrent hemoptysis and purulent sputum with a normal chest X-ray suggest a possible diagnosis of bronchiectasis or a pulmonary infection, despite the lack of obvious radiographic findings.

**Why the Correct Answer is Right**
The next best investigation for Rampal would be a high-resolution computed tomography (HRCT) scan of the lungs. HRCT provides detailed images of the lung parenchyma, bronchi, and bronchioles, which can help identify bronchiectasis, cystic changes, or other abnormalities that may not be visible on a standard chest X-ray. This would aid in diagnosing the underlying cause of his symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** A bronchoscopy might be performed in certain situations, but it's not the next best investigation for Rampal, given his normal chest X-ray. It's more invasive and usually reserved for cases where a specific diagnosis is suspected or when other tests are inconclusive.
**Option B:** A pulmonary function test (PFT) would provide information on lung function but would not directly address the cause of hemoptysis or purulent sputum.
**Option C:** A ventilation-perfusion scan is typically used to diagnose pulmonary embolism, which is not the primary concern in this case.
**Option D:** A magnetic resonance imaging (MRI) of the chest is not the first-line investigation for evaluating lung parenchyma or bronchi.

**Clinical Pearl / High-Yield Fact**
When evaluating patients with recurrent hemoptysis, always consider the possibility of bronchiectasis, and HRCT is the imaging modality of choice for diagnosing this condition.

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