**Core Concept:** Pterygium is a benign, fibrovascular growth on the conjunctiva that can invade the cornea and potentially cause visual disturbances. Surgical treatment is often necessary for symptomatic relief and prevention of corneal involvement.

**Why the Correct Answer is Right:** The most common problem following surgical treatment of pterygium is a complication known as "epithelial ingrowth." This occurs when the conjunctival epithelium from the surgical wound edges or adjacent conjunctiva migrates into the cornea, potentially leading to visual disturbances, inflammation, and delayed healing.

**Why Each Wrong Option is Incorrect:**

A. **Irritation and inflammation:** While inflammation is a common postoperative complication, it is not the most common issue following pterygium surgery. Epithelial ingrowth is the primary concern.

B. **Corneal perforation:** This is an extremely rare complication, and other options are more likely causes of postoperative problems.

C. **Recurrence:** Although recurrence is a concern after pterygium surgery, the correct answer is not recurrence itself, but rather the related complication of epithelial ingrowth.

D. **Corneal ulcer:** Although postoperative corneal ulcers can occur, they are not the most common issue following pterygium surgery. Epithelial ingrowth is the primary concern.

**Clinical Pearl:** In cases of pterygium surgery, proper surgical technique and management of the conjunctival-corneal interface are crucial to minimize the risk of epithelial ingrowth and other common postoperative complications.

**Correct Answer:** D. Corneal ulcer

**Explanation:** While corneal ulcers can occur postoperatively, they are not the most common problem following pterygium surgery. The primary concern is the ingrowth of conjunctival epithelium into the cornea, which can lead to symptoms, inflammation, and delayed healing. Corneal ulcers are a consequence of this ingrowth and not the primary issue.