**Core Concept**
Pityriasis rosea is a skin condition characterized by an initial annular herald patch followed by a secondary eruption of oval-shaped, scaly patches. The pathogenesis of pityriasis rosea is not fully understood, but it is thought to result from a viral infection, possibly human herpesvirus 6 or 7, leading to an immune response and skin manifestations.

**Why the Correct Answer is Right**
The annular herald patch is a distinctive feature of pityriasis rosea, typically appearing 1-2 weeks before the secondary eruption. It is usually a solitary, oval-shaped patch with a clear center and a scaly, erythematous border. The herald patch is thought to be a precursor to the secondary eruption, which can involve multiple areas of the body. The exact mechanism of the herald patch is unclear, but it is believed to result from a localized immune response to the viral infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Pityriasis alba is a mild, chronic skin condition characterized by small, round, scaly patches on the face, typically in children. It does not have an annular herald patch.

**Option C:** Tinea capitis is a fungal infection of the scalp, characterized by hair loss, scaling, and sometimes pus-filled bumps. It does not have an annular herald patch.

**Option D:** Psoriasis is a chronic skin condition characterized by thick, scaly plaques, often on the elbows, knees, and scalp. It does not have an annular herald patch.

**Clinical Pearl / High-Yield Fact**
The "mother patch" or herald patch of pityriasis rosea is often located on the trunk, and the secondary eruption typically follows 1-2 weeks later, involving multiple areas of the body.

**✓ Correct Answer: B. Pityriasis rosea**