**Core Concept**
The most common infection in contact lens users is caused by the colonization and proliferation of microorganisms on the lens, leading to a compromised ocular surface. This is often due to poor lens hygiene, inadequate disinfection, or the use of contaminated contact lens solution.

**Why the Correct Answer is Right**
The correct answer is **A. Pseudomonas aeruginosa**. Pseudomonas aeruginosa is a gram-negative rod-shaped bacterium that thrives in moist environments, making it an ideal candidate for causing contact lens-related infections. It can produce virulence factors that allow it to adhere to and colonize the contact lens surface, leading to corneal ulcers and other ocular complications.

**Why Each Wrong Option is Incorrect**
* **Option B:** Fungal keratitis is a serious infection that can occur in contact lens users, but it is less common than bacterial infections like Pseudomonas aeruginosa. Fungal keratitis is often associated with trauma to the eye or a compromised ocular surface.
* **Option C:** Acute angle-closure glaucoma is a medical emergency that requires prompt treatment, but it is not directly related to contact lens use. It is a condition where the iris blocks the drainage angle of the eye, leading to increased intraocular pressure.
* **Option D:** Herpes simplex keratitis is a viral infection that can cause corneal ulcers and other ocular complications, but it is not the most common infection in contact lens users. Herpes simplex keratitis is often caused by reactivation of latent virus.

**Clinical Pearl / High-Yield Fact**
It's essential for contact lens users to follow proper hygiene and disinfection protocols to prevent the colonization of microorganisms on the lens. This includes cleaning the lens with sterile solution, rinsing with sterile water, and storing the lens in a clean case.

**Correct Answer:** A. Pseudomonas aeruginosa