**Core Concept**
Erythroderma, also known as exfoliative dermatitis, is a rare, severe skin condition characterized by widespread scaling and erythema involving more than 90% of the skin surface. The condition is often associated with underlying systemic diseases, medications, or malignancies.

**Why the Correct Answer is Right**
The key to diagnosing erythroderma lies in the extent of skin involvement. The diagnosis is typically made when more than 90% of the skin surface is affected, which can be estimated by assessing the affected body surface area (BSA). This involves calculating the percentage of skin involved using the "rule of nines," which assigns specific percentages to different body regions. In erythroderma, the affected areas often include the face, trunk, and limbs, leading to the characteristic widespread erythema and scaling.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might be tempting for students who are not familiar with the specific criteria for diagnosing erythroderma. However, it is generally accepted that the skin involvement should be more extensive than 50% to meet the diagnostic criteria.

**Option B:** This option might be considered by students who are thinking about other skin conditions that can cause widespread erythema, such as psoriasis or eczema. However, these conditions typically do not involve more than 90% of the skin surface.

**Option C:** This option might be considered by students who are thinking about the severity of skin involvement, rather than the extent. However, the severity of skin involvement does not determine the diagnosis of erythroderma.

**Clinical Pearl / High-Yield Fact**
To remember the diagnostic criteria for erythroderma, the "9" in "rule of nines" can be used as a mnemonic to recall that more than 90% of the skin surface must be involved.

**Correct Answer: C. 90%**