**Core Concept**
The patient's symptoms suggest an acute inflammatory condition affecting the eye, specifically involving the adnexa, which includes the eyelids, lacrimal gland, and surrounding tissues.

**Why the Correct Answer is Right**
The patient's presentation of sudden onset, unilateral, painful, red eye is highly suggestive of acute angle-closure glaucoma. This condition occurs when the iris suddenly blocks the drainage angle of the eye, leading to a rapid increase in intraocular pressure (IOP). The pain, nausea, vomiting, and headache are due to increased IOP causing pressure on surrounding structures.

The increased IOP is mediated by the obstruction of aqueous humor outflow, which is normally facilitated by the trabecular meshwork and the uveoscleral pathway. The obstruction leads to a buildup of aqueous humor, causing the IOP to rise rapidly. This can lead to optic nerve damage and vision loss if not promptly treated.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while a red eye can be a symptom of conjunctivitis, the combination of nausea, vomiting, and headache is not typical for this condition.

**Option B:** This option is incorrect because a subconjunctival hemorrhage presents as a localized collection of blood under the conjunctiva, but it does not typically cause pain, nausea, or headache.

**Option C:** This option is incorrect because an orbital cellulitis would typically present with more systemic symptoms such as fever and would not be limited to a single eye.

**Clinical Pearl / High-Yield Fact**
Acute angle-closure glaucoma is a medical emergency that requires prompt treatment to prevent vision loss. The treatment typically involves administering intravenous acetazolamide to reduce IOP, followed by topical medications and possibly surgical intervention.

**Correct Answer: A. Acute angle-closure glaucoma**