Herald Patch and Rash Diagnosis

The question asks to identify the condition characterized by a 'herald patch' followed by a widespread 'rash'. This specific sequence is a classic diagnostic feature.

Pityriasis Rosea Explained

Pityriasis rosea is a common, self-limiting skin condition. Its presentation typically includes:

• A single, larger oval patch called the herald patch, which appears first.
• A subsequent eruption of smaller, similar oval patches (the rash) appearing days to weeks later, often on the trunk and proximal extremities.
• The secondary rash often follows the skin lines of cleavage, creating a characteristic 'Christmas tree' pattern on the back.

This pattern directly matches the description provided in the question.

Differential Diagnosis

Let's consider why the other options are less likely:

• Psoriasis: Typically presents as well-defined, erythematous plaques with thick silvery scales. It does not usually start with a herald patch.
• Lichen planus: Characterized by the 4 Ps - Pruritic, Purple, Polygonal, Papules and Plaques. It does not involve a herald patch preceding a generalized rash.
• Dermatophytosis (Fungal infection): Often presents as annular (ring-shaped) lesions with central clearing and scaly borders. While it can cause a rash, the herald patch is not a typical feature.

Therefore, the presence of a herald patch followed by a rash is most characteristic of Pityriasis rosea.