**Core Concept**
The most definitive diagnosis of sinusitis involves direct visualization of the sinus cavities to assess inflammation, polyps, or obstruction. Unlike imaging or indirect tests, sinuscopy provides real-time, anatomical insight into the sinus lining and patency.

**Why the Correct Answer is Right**
Sinuscopy (nasal endoscopy) allows direct visualization of the sinus openings and mucosal lining, enabling accurate assessment of inflammation, polyps, or obstruction. It is considered the gold standard for diagnosing acute or chronic sinusitis because it reveals the actual pathology, such as mucosal swelling or secretions, which imaging may miss. This procedure is performed under local anesthesia and is both sensitive and specific.

**Why Each Wrong Option is Incorrect**
Option A: X-ray PNS (Paranasal Sinuses) is outdated and insensitive; it shows bone changes late in disease and lacks sensitivity for early or mild sinusitis.
Option B: Proof puncture (a distractor; likely meant as "puncture" or "aspiration") is not a standard or safe diagnostic method for sinusitis and carries infection risk without diagnostic value.
Option D: Transillumination test is a screening tool only, not definitive; it detects air-filled sinuses but cannot confirm inflammation or pathology.

**Clinical Pearl / High-Yield Fact**
Always perform sinuscopy when diagnosing suspected sinusitis—especially in chronic or atypical cases—to avoid misdiagnosis and ensure appropriate management. It is the only test that directly visualizes the sinus mucosa and is essential before initiating antibiotics or surgical intervention.

✓ Correct Answer: C. Sinuscopy