**Core Concept:** Stricture esophagus is a narrowing of the esophagus caused by injury or damage to its inner lining. In cases of corrosive poisoning, investigations are required to assess the extent and severity of the injury, identify complications, and guide management.

**Why the Correct Answer is Right:** In this scenario, the correct answer is **Option C: Endoscopy (Esophagoscopy)**. Endoscopy allows direct visualization of the esophagus, evaluating the extent and severity of the injury, identification of complications such as ulcers, perforations, or strictures. It is crucial for treatment planning, guiding dilatation procedures, and assessing the response to treatment.

**Why Each Wrong Option is Incorrect:**

A. **Option A: Barium swallow (X-ray examination)** is helpful in identifying the level of stricture but does not provide detailed information about the extent and severity of the injury or any associated complications.

B. **Option B: Computed tomography (CT scan)** is useful for detecting complications like perforation or mediastinitis but does not give direct visualization of the esophagus for assessing the stricture.

D. **Option D: Upper GI endoscopy (gastroscopy)** is appropriate for evaluating the stomach and duodenum but does not adequately assess the esophagus and the stricture caused by corrosive poisoning.

**Clinical Pearl:** Esophagoscopy is a gold standard investigation for corrosive esophageal stricture. It guides management decisions, such as dilatation, surgery, or combination therapy. Corrosive esophageal stricture often requires regular follow-up endoscopy to assess response to treatment and avoid complications like perforation or rupture during dilatation.