**Core Concept**
Toxic epidermal necrolysis (TEN) is a severe skin condition characterized by widespread necrosis of the epidermis, often triggered by medications. It is a variant of Stevens-Johnson syndrome (SJS), which is a form of mucocutaneous lymph node syndrome. The pathophysiology of TEN involves a dysregulated immune response, leading to apoptosis of keratinocytes and subsequent detachment of the epidermis.

**Why the Correct Answer is Right**
The body surface area (BSA) affected in TEN is a crucial factor in determining the severity and mortality of the condition. The BSA is typically estimated using the Rule of Nines, which assigns a percentage of BSA to different body regions. In TEN, the BSA affected is usually more than 30%, and in some cases, it can be as high as 90%. This widespread epidermal detachment can lead to significant fluid loss, electrolyte imbalances, and increased susceptibility to infections.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the BSA affected in TEN is not a direct measure of the severity of the condition. While a larger BSA affected may indicate a worse prognosis, the extent of epidermal necrosis is also a critical factor.
**Option B:** This option is incorrect because the BSA affected in TEN is not a direct measure of the mortality rate. While a larger BSA affected may be associated with a higher mortality rate, other factors such as age, comorbidities, and the presence of systemic involvement also play a role.

**Clinical Pearl / High-Yield Fact**
In TEN, the BSA affected is a crucial factor in determining the severity of the condition, and a BSA of more than 30% is generally considered a poor prognostic indicator.

**Correct Answer:** B. The body surface area affected in TEN is typically more than 30%.