**Core Concept**
SLE (Systemic Lupus Erythematosus) is a chronic autoimmune disease characterized by the presence of autoantibodies and inflammation in multiple organs, including the skin. Cutaneous manifestations are common in SLE and can range from mild to severe.

**Why the Correct Answer is Right**
The most common cutaneous eruption in SLE is the malar rash, also known as a "butterfly rash". This occurs due to the deposition of immune complexes and complement activation in the dermal-epidermal junction, leading to inflammation and vasculitis. The malar rash typically spares the nasolabial folds and can be exacerbated by sun exposure.

**Why Each Wrong Option is Incorrect**
* **Option A:** Discoid lupus is a type of cutaneous lupus erythematosus characterized by coin-shaped lesions on the face and scalp, but it is not the most common cutaneous eruption in SLE.
* **Option B:** Photosensitivity is a common feature of SLE, but it is not a specific cutaneous eruption.
* **Option C:** Vasculitis is a complication of SLE, but it is not a specific type of cutaneous eruption.

**Clinical Pearl / High-Yield Fact**
The "butterfly rash" of SLE is often described as a "ragged, erythematous rash" that spans the cheeks and nose, and is typically worse in sun-exposed areas.

**Correct Answer:** C.