**Core Concept:**
The presented clinical scenario involves a patient with signs and symptoms suggestive of an ocular infection, specifically, an orbital cellulitis. Orbital cellulitis is an infection that involves the soft tissues of the orbit, which is the bony cavity that houses the eye. In this case, the patient presents with pain, swelling, tenderness, and purulent discharge, along with normal visual acuity.

**Why the Correct Answer is Right:**
The correct answer (D) is based on the clinical findings described in the question. Orbital cellulitis often presents with symptoms like pain, swelling, and purulent discharge from the affected eye. In addition, tenderness and edema over the medial canthus are common signs of this condition. Although the patient's visual acuity is normal, this is not a definitive feature as orbital cellulitis can initially spare the visual acuity due to the relative sparing of the optic nerve early in the course of the disease.

**Why Each Wrong Option is Incorrect:**
A. Infectious keratitis: This option is incorrect because the patient does not present with signs of corneal involvement (e.g., pain on eye movement, corneal edema, or hypopyon).

B. Hyphema: Hyphema is a condition characterized by blood in the anterior chamber of the eye. The patient's normal visual acuity and normal intraocular pressure (not mentioned in the question) rules out this diagnosis.

C. Hypotony: Hypotony is characterized by low intraocular pressure. The normal intraocular pressure mentioned in the question rules out this diagnosis.

D. Orbital cellulitis is the correct answer as it matches the clinical findings described in the question, including pain, swelling, and purulent discharge from the affected eye. The patient's normal visual acuity does not exclude the possibility of orbital cellulitis, as visual acuity can be preserved early in the disease course.

**Clinical Pearl:**
In cases where a patient presents with ocular symptoms and signs, it is essential to consider orbital cellulitis in the differential diagnosis. Although visual acuity is often preserved initially, prompt initiation of appropriate treatment is crucial, as orbital cellulitis can rapidly progress to threaten vision and cause significant complications if not treated promptly.