**Core Concept**
The question is testing the student's knowledge of mucocutaneous manifestations of autoimmune diseases, specifically the association of lacy linear lesions with systemic sclerosis.

**Why the Correct Answer is Right**
The patient's presentation of lacy linear lesions on the buccal mucosa, also known as "lacy pattern", is a characteristic feature of systemic sclerosis (scleroderma). This is due to the deposition of collagen in the dermal layer of the mucosa. The elongation of the nail fold beyond the nail bed, also known as sclerodactyly, is another hallmark of this disease. Systemic sclerosis is a chronic autoimmune disease characterized by fibrosis and vascular alterations in the skin and internal organs.

**Why Each Wrong Option is Incorrect**

* **Option A:** This option is incorrect because it does not match the description of the patient's symptoms. The patient's presentation is not consistent with lichen planus, which typically presents with a reticular, polygonal, or Wickham's striae pattern on the buccal mucosa.
* **Option B:** This option is incorrect because it is not a common manifestation of autoimmune diseases. The patient's presentation is not consistent with pemphigus vulgaris, which typically presents with blistering lesions on the skin and mucous membranes.
* **Option C:** This option is incorrect because it is not a characteristic feature of autoimmune diseases. The patient's presentation is not consistent with oral lichen planus, which typically presents with a reticular, polygonal, or Wickham's striae pattern on the buccal mucosa.

**Clinical Pearl / High-Yield Fact**
The "lacy pattern" on the buccal mucosa is a characteristic feature of systemic sclerosis, and its presence should raise suspicion for this disease, especially in patients with sclerodactyly and other systemic manifestations.

**Correct Answer:** C.