**Core Concept**
Lichen planus is a mucocutaneous disease that primarily affects the skin and mucous membranes, characterized by an autoimmune response to basal cell layer antigens. This results in a T-cell mediated inflammatory response, leading to the characteristic lesions.

**Why the Correct Answer is Right**
The correct answer is B. Lichen planus because it is a disease that predominantly affects the mucous membranes, including the oral mucosa, esophagus, and genital mucosa. The oral mucosa is the most common site affected, manifesting as white, lacy patches or lesions. The involvement of the mucosa in lichen planus is due to the autoimmune response targeting the basal cell layer of the mucous membrane, leading to inflammation and damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Psoriasis is primarily a skin condition characterized by thick, scaly plaques, but it can also involve the nails and joints. While psoriasis can occasionally involve mucous membranes, it is not a mucocutaneous disease like lichen planus.

**Option C:** Alopecia refers to hair loss, which can be due to various causes, including autoimmune diseases, genetics, and hormonal imbalances. Alopecia is not a mucocutaneous disease and does not involve the mucous membranes.

**Option D:** Scabies is a skin infestation caused by the mite Sarcoptes scabiei, leading to intense itching and a pustular rash. While scabies can involve the genital area, it is not a mucocutaneous disease and does not primarily affect the mucous membranes.

**Clinical Pearl / High-Yield Fact**
A key feature of lichen planus is the presence of Wickham's striae, which are white, lacy lines that appear on the surface of the lesions. These striae are a characteristic finding in oral lichen planus and can help in diagnosing the condition.

**✓ Correct Answer: B. Lichen planus**