**Core Concept**
Photophobia and rapid-onset corneal ulcer with a greenish, necrotic base are hallmark signs of bacterial keratitis, most commonly due to *Pseudomonas aeruginosa*. This organism is highly virulent, rapidly invasive, and commonly associated with trauma or contact lens wear, leading to severe inflammation and corneal destruction.

**Why the Correct Answer is Right**
*Pseudomonas aeruginosa* is the most common causative agent of severe bacterial keratitis. It produces greenish pus due to pigments like pyocyanin and exhibits rapid growth, leading to necrotic ulcers with intense photophobia. It is a Gram-negative bacillus that invades corneal tissue, causing acute inflammation and rapid corneal sloughing. Its presence is confirmed by Gram stain showing gram-negative rods with greenish discoloration of the ulcer base.

**Why Each Wrong Option is Incorrect**
Option A: *Acanthamoeba* causes chronic, recurrent, and often progressive keratitis, typically in contact lens wearers, but presents with slow progression and a grayish or white base, not greenish. It does not cause acute photophobia within 24 hours.
Option B: *Nocardia* is a rare, slow-growing organism that causes chronic, subacute infections, often in immunocompromised patients, with a granulomatous appearance and no acute photophobia.
Option D: Herpes simplex virus causes recurrent, recurrent epithelial ulcers with dendritic patterns, typically in the presence of viral reactivation, and does not present with greenish ulcers or rapid sloughing.

**Clinical Pearl / High-Yield Fact**
In acute, painful, greenish corneal ulcers with photophobia, *Pseudomonas* is the top differential—prompt antibiotic therapy (e.g., topical ciprofloxacin or tobramycin) is essential to prevent vision loss.

✓ Correct Answer: C. Pseudomonas