**Core Concept**
Posterior subcapsular cataract (PSC) is a type of age-related cataract characterized by the opacification of the posterior subcapsular region of the lens. This condition can be caused by various factors, including genetic predisposition, aging, and exposure to certain medications or radiation.

**Why the Correct Answer is Right**
Posterior subcapsular cataract can be caused by prolonged use of corticosteroids, which induce oxidative stress and damage to the lens epithelial cells. The exact mechanism involves the disruption of the normal metabolic processes, leading to the accumulation of advanced glycosylation end-products (AGEs) and the activation of various signaling pathways that contribute to the development of cataract. In addition, radiation exposure, such as from cumulative doses of UV radiation or ionizing radiation, can also cause PSC by inducing DNA damage and oxidative stress in the lens epithelial cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because radiation exposure is a known cause of posterior subcapsular cataract. This is supported by studies showing an increased risk of cataract development in individuals exposed to ionizing radiation.

**Option B:** Incorrect because prolonged use of corticosteroids is a well-established risk factor for posterior subcapsular cataract. This is due to the steroid-induced oxidative stress and damage to the lens epithelial cells.

**Option C:** Incorrect because this option is a placeholder and does not provide a valid choice.

**Clinical Pearl / High-Yield Fact**
It's essential to note that posterior subcapsular cataract can also be caused by other factors, including genetic predisposition, age-related changes, and exposure to smoking and UV radiation. Regular eye examinations are crucial in detecting cataracts early, allowing for timely intervention and prevention of vision loss.

**Correct Answer: D**