**Core Concept**
Orange-skin cornea is a characteristic corneal lesion resulting from exposure to specific chemical agents. This condition represents an inflammatory response to toxic substances, leading to corneal edema and subsequent formation of characteristic corneal lesions.

**Why the Correct Answer is Right**
Mustard gas, a chemical warfare agent, causes severe eye damage upon exposure. The cornea becomes edematous and develops characteristic "orange peel" or "maple syrup" spots due to the toxic effects of mustard gas on the corneal epithelium and stroma. The mechanism involves the release of inflammatory mediators and the disruption of cellular structures, leading to the characteristic corneal lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** Chalcosis refers to the deposition of copper in the cornea, typically resulting from the presence of an intraocular foreign body. This condition causes a different set of clinical manifestations, including the Kayser-Fleischer ring and sunflower cataract.
**Option B:** Siderosis is a condition caused by the deposition of iron in the cornea, usually resulting from the presence of an intraocular iron-containing foreign body. This condition also has distinct clinical features, including corneal iron deposition and visual disturbances.
**Option C:** Ammonia burn causes severe corneal damage, but the resulting lesions are typically more severe and widespread than those seen in mustard gas exposure. Ammonia burns often result in corneal perforation or severe vascularization.

**Clinical Pearl / High-Yield Fact**
Mustard gas exposure can cause systemic symptoms, including respiratory distress and skin lesions, in addition to the characteristic corneal lesions. This highlights the importance of considering the patient's overall clinical presentation when evaluating eye damage.

**✓ Correct Answer: D. Mustard gas**