**Core Concept**
Lichen planus is a chronic inflammatory condition affecting the skin, mucous membranes, and nails, characterized by a characteristic violaceous papular rash, pterygium of the nail, and cicatricial alopecia.

**Why the Correct Answer is Right**
The patient's symptoms are classic for lichen planus, an autoimmune condition mediated by T-cell activation and the release of cytokines, which leads to inflammation and tissue damage. The disease involves the deposition of immune complexes in the dermis, resulting in the characteristic violaceous papules and pterygium of the nail. Cicatricial alopecia occurs due to the destruction of hair follicles by the inflammatory infiltrate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because lichen planus is not primarily caused by a bacterial or viral infection, but rather by an autoimmune response.
**Option B:** This option is incorrect because psoriasis is a separate condition characterized by erythematous plaques with silvery scales, not violaceous papules or pterygium of the nail.
**Option C:** This option is incorrect because discoid lupus erythematosus is a type of lupus that affects the skin, but it typically presents with erythematous plaques and scarring, not violaceous papules or pterygium of the nail.

**Clinical Pearl / High-Yield Fact**
Lichen planus can affect multiple organ systems, including the skin, mucous membranes, nails, and even the oral cavity, making it a diagnosis that requires a thorough examination of all these areas.

**Correct Answer:** C. Lichen planus