**Core Concept**
The earliest symptom of a corneal ulcer is pain due to the release of pro-inflammatory mediators and the activation of nociceptors in the cornea. This pain is often described as a sharp, stabbing, or burning sensation and is usually the first sign of corneal ulceration.

**Why the Correct Answer is Right**
The pain associated with a corneal ulcer is primarily caused by the activation of nociceptors in the cornea, which are sensitive to pro-inflammatory mediators such as bradykinin, serotonin, and histamine. These mediators are released in response to the presence of an ulcer and stimulate the nociceptors to send pain signals to the brain. The cornea is richly innervated with nociceptors, which makes it highly sensitive to pain-inducing stimuli. As a result, pain is typically the first symptom to occur in a corneal ulcer.

**Why Each Wrong Option is Incorrect**
**Option B:** Photophobia (sensitivity to light) may occur later in the course of a corneal ulcer, as the ulceration leads to inflammation and scarring of the cornea, which can affect the cornea's ability to regulate light entry into the eye.
**Option C:** Loss of sensation is not typically an early symptom of a corneal ulcer, as the ulceration initially stimulates the nociceptors, leading to pain, rather than causing a loss of sensation.
**Option D:** Diminished vision may occur later in the course of a corneal ulcer, as the ulceration leads to scarring and opacification of the cornea, which can impair vision.

**Clinical Pearl / High-Yield Fact**
A classic exam question trap is to confuse the symptoms of a corneal ulcer with those of a corneal abrasion. While both conditions can cause pain, a corneal abrasion typically causes a more localized area of pain and is often associated with a history of trauma to the eye.

**✓ Correct Answer: A. Pain**