**Core Concept**
Lichen planus is a chronic, inflammatory, and autoimmune skin disease characterized by an interface dermatitis, where the immune system attacks the skin's basal cell layer. Nail involvement is common, manifesting as various changes to the nail plate and underlying nail apparatus.

**Why the Correct Answer is Right**
The characteristic nail finding in lichen planus is the presence of **pterygium**, which is a central or lateral wing-like scarring of the nail bed, leading to nail plate thinning and eventual destruction. This occurs due to chronic inflammation and scarring of the nail matrix and nail bed, resulting from the autoimmune response against the skin and nail tissues. The inflammation and subsequent scarring cause the nail plate to become adherent to the nail bed, leading to the characteristic wing-like appearance.

**Why Each Wrong Option is Incorrect**
**Option A:** **Ridges** are a non-specific finding that can be seen in various nail disorders, including psoriasis, eczema, and trauma. While ridging can be a feature of lichen planus, it is not the characteristic finding.
**Option B:** **Clubbing** is a sign of underlying systemic disease, such as lung cancer, heart disease, or cystic fibrosis, and is not a feature of lichen planus.
**Option C:** **Onycholysis** is the separation of the nail plate from the nail bed, seen in conditions such as psoriasis, trauma, or fungal infections. While onycholysis can occur in lichen planus, it is not the characteristic finding.

**Clinical Pearl / High-Yield Fact**
Lichen planus can involve multiple systems, including the skin, mucous membranes, nails, and even the oral cavity, making it a systemic disease that requires a comprehensive approach to diagnosis and management.

**Correct Answer: D. Pterygium**