**Core Concept**
Exanthems are a group of skin eruptions that are a manifestation of an underlying systemic disease. They are often caused by viral infections, but can also be triggered by bacterial, fungal, or other factors. Exanthems are a key diagnostic clue for various systemic conditions.

**Why the Correct Answer is Right**
Exanthems can be caused by a variety of mechanisms, including viral replication in the skin, immune complex deposition, and altered cell-mediated immunity. For example, measles is caused by the measles virus, which replicates in the skin and causes a characteristic rash. Rubella is another viral exanthem caused by the rubella virus. On the other hand, Stevens-Johnson syndrome is a severe form of exanthem caused by an immune-mediated reaction to medications or infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Not given, but we'll assume it's a plausible option.
**Option B:** Incorrect – some viral exanthems are caused by viral infections, such as measles and rubella.
**Option C:** Incorrect – some bacterial exanthems are caused by bacterial infections, such as erythema infectiosum caused by Parvovirus B19.
**Option D:** Incorrect – some fungal exanthems are caused by fungal infections, such as pityriasis versicolor caused by Malassezia.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with an exanthem, it's essential to consider the patient's exposure history, travel history, and potential underlying conditions that may be triggering the exanthem.

**Correct Answer:** B.