A patient has eye pain, blurred vision, and a dendritic ulcer. What is the most likely diagnosis?

The presence of eye pain, blurred vision, and a dendritic ulcer strongly suggests a diagnosis of viral keratitis, specifically herpes simplex keratitis. Dendritic ulcers are characteristic findings associated with this condition, representing the corneal epithelium damage caused by the herpes simplex virus.

Viral keratitis often presents with symptoms such as redness, pain, photophobia, and blurred vision, all of which align with this patient's symptoms. Dendritic ulcers can be visualized using fluorescein staining, where they appear as branching, tree-like patterns on the cornea, confirming involvement of the herpes simplex virus.

Other conditions may present with corneal ulcers, but they typically have distinct characteristics that differentiate them from viral keratitis. For instance, fungal keratitis and bacterial corneal ulcers usually produce more diffuse, opacity-containing lesions and may have accompanying signs of severe intraocular inflammation, while Acanthamoeba keratitis has different risk factors and presents with severe pain and ring-shaped infiltrates on the cornea. Thus, the combination of symptoms and the specific appearance of the dendritic ulcer point to viral keratitis as the most likely diagnosis.