**Core Concept**
The most common cause of cataract is a complex interplay of factors involving aging, oxidative stress, and accumulation of metabolic byproducts that lead to lens protein denaturation and aggregation. This process ultimately results in the characteristic opacity of the lens that impairs vision.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of cataract formation, where a significant role is played by the accumulation of advanced glycosylation end-products (AGEs) and other oxidative stress markers. These molecules lead to the formation of cross-links between lens proteins, causing their denaturation and aggregation, which ultimately results in cataract formation. This process is accelerated by factors such as ultraviolet radiation, smoking, and diabetes mellitus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it is a rare congenital cause of cataract, not the most common cause.
**Option B:** This option is incorrect as it is associated with a specific type of cataract, but not the most common cause overall.
**Option C:** This option is incorrect as it is a metabolic disorder that can contribute to cataract formation, but not the most common cause.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that cataract is a multifactorial disease, and a comprehensive approach involving both pharmacological and non-pharmacological interventions is necessary to prevent and manage cataract formation.

**Correct Answer: C. Diabetes mellitus.**