**Core Concept:** Pterygium is a benign fibrovascular growth on the conjunctiva that extends into the cornea. Progressive pterygium refers to the gradual expansion of this growth over time.

**Why the Correct Answer is Right:** The correct answer, D, refers to the clinical manifestation of progressive pterygium, which is characterized by the growth of the pterygium into the cornea, causing visual disturbances, discomfort, and potential corneal erosion. This leads to pain, photophobia, and foreign body sensation. The other options do not accurately describe the effects of progressive pterygium.

**Why Each Wrong Option is Incorrect:**

A. This option does not mention the progressive nature of the pterygium and its consequences on vision.
B. Although pterygium can cause redness, the correct answer D is more specific to progressive pterygium.
C. Pain and photophobia are not the primary symptoms of pterygium but can occur due to the corneal involvement by the progressive growth.

**Clinical Pearl:** In cases of progressive pterygium, timely intervention (e.g., surgery) is crucial to prevent potential complications like corneal opacity, ulceration, and even blindness.

**Correct Answer:** D. Corneal involvement and visual disturbances