**Core Concept**
Onion peel appearance cataract refers to a type of cataract characterized by a layer of cortical opacities that resemble the layers of an onion. This type of cataract is caused by the deposition of cortical proteins and lipids, leading to the formation of a characteristic layered or striated appearance.

**Why the Correct Answer is Right**
The onion peel appearance cataract is typically seen in patients with diabetes mellitus. The underlying pathophysiology involves the accumulation of sorbitol in the lens due to increased glucose levels. Sorbitol accumulation leads to osmotic changes, causing water to move into the lens fibers, resulting in cortical lens opacification. This process is exacerbated by the presence of advanced glycosylation end-products (AGEs) that accumulate in the lens and contribute to the formation of the characteristic onion peel appearance.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because posterior subcapsular cataract is a different type of cataract characterized by an opacity in the posterior subcapsular region of the lens.
**Option B:** This option is incorrect because cortical cataract is a broader category that encompasses several types of cataract, including the onion peel appearance, but is not specific to the characteristic layered appearance of this type of cataract.
**Option C:** This option is incorrect because zonular cataract is a type of cataract that involves the zonular fibers of the lens and is not typically associated with the onion peel appearance.

**Clinical Pearl / High-Yield Fact**
Onion peel cataract is a classic example of a cortical cataract that is associated with diabetes mellitus. It is essential to recognize this type of cataract in patients with diabetes, as it may be an early sign of underlying metabolic disorders.

**Correct Answer:** C.