**Core Concept**
Alopecia areata is an autoimmune disease characterized by localized patches of complete hair loss due to the immune system attacking the hair follicles. It typically presents with well-demarcated, round patches of hair loss on the scalp, often with a normal appearance of the surrounding skin.

**Why the Correct Answer is Right**
In alopecia areata, the immune system mistakenly identifies the hair follicles as foreign and launches an immune response against them. This results in the destruction of the hair follicles, leading to the characteristic patches of hair loss. The disease is often associated with other autoimmune conditions, such as thyroid disease or vitiligo. The immune cells, including T-lymphocytes and macrophages, infiltrate the hair follicles and cause inflammation, leading to the loss of hair. The disease can be triggered by stress, infections, or other environmental factors.

**Why Each Wrong Option is Incorrect**
**Option A:** Tinea capitis is a fungal infection of the scalp, typically presenting with patches of hair loss, but the scalp itself is often inflamed and may have a characteristic "black dot" appearance due to the fungal spores.

**Option B:** Cradle cap, also known as seborrhoeic dermatitis, is a skin condition characterized by a flaky, oily scalp, but it does not typically cause complete hair loss.

**Option D:** Telogen effluvium is a condition characterized by excessive hair shedding, often due to stress or hormonal changes, but it typically presents with diffuse hair loss rather than localized patches.

**Clinical Pearl / High-Yield Fact**
Alopecia areata can be a sign of an underlying autoimmune disease, and patients may also exhibit other symptoms such as vitiligo, thyroid disease, or autoimmune hemolytic anemia.

**✓ Correct Answer: C. Alopecia areata**