**Core Concept**
The patient's presentation of haemoptysis, upper respiratory symptoms, and cavitating nodules on CT scan suggests a diagnosis of a systemic disease affecting the lungs and other organs. The patient's history of frequent epistaxis and purulent discharge suggests a possible underlying condition leading to chronic inflammation and tissue damage.

**Why the Correct Answer is Right**
The patient's symptoms are consistent with Granulomatosis with Polyangiitis (GPA), a form of vasculitis affecting small to medium-sized vessels. The cavitating nodules on CT scan and the presence of RBCs in the urine suggest kidney involvement. The correct test to make the diagnosis is a test for anti-neutrophil cytoplasmic antibodies (ANCA), specifically proteinase 3 (PR3) ANCA, which is highly sensitive and specific for GPA. This test can help differentiate GPA from other forms of vasculitis and autoimmune disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** A chest X-ray would not provide sufficient information to make a diagnosis of GPA, as it would only show non-specific findings such as infiltrates or nodules.
**Option B:** A bronchoscopy with biopsy may be helpful in evaluating the lung lesions, but it would not provide a definitive diagnosis of GPA. Additionally, it carries a risk of complications such as bleeding or pneumothorax.
**Option C:** An echocardiogram may be useful in evaluating cardiac involvement, but it would not provide a diagnosis of GPA.
**Option D:** A complete blood count (CBC) may show anemia or leukocytosis, but it would not provide a specific diagnosis of GPA.

**Clinical Pearl / High-Yield Fact**
GPA is a form of vasculitis that affects the respiratory tract and kidneys, and is associated with the presence of PR3 ANCA. Remember to consider GPA in patients with chronic respiratory symptoms, haemoptysis, and kidney involvement.

**Correct Answer:** C. ANCA (anti-neutrophil cytoplasmic antibodies) testing, specifically for proteinase 3 (PR3) ANCA.