**Core Concept**
Pleomorphism in the context of viral exanthems refers to the diverse range of skin manifestations that can occur during an infection. This phenomenon is characterized by the presence of various skin lesions of different sizes, shapes, and stages of development simultaneously.

**Why the Correct Answer is Right**
Chickenpox, caused by the Varicella-Zoster virus, is a classic example of pleomorphic skin lesions. The rash typically begins as flat, pink spots that progress to papules, vesicles, pustules, and eventually crusts over. The simultaneous presence of these different stages of skin lesions is a hallmark of pleomorphism. This characteristic rash is a result of the virus's ability to infect and replicate in different layers of the skin, leading to the varied appearance of the lesions.

**Why Each Wrong Option is Incorrect**
**Option B:** Rubella, caused by the Rubella virus, typically presents with a monomorphic rash, characterized by a uniform, pink, maculopapular eruption.

**Option C:** Smallpox, caused by the Variola virus, is known for its distinctive, uniform rash, which is a key feature in distinguishing it from other viral exanthems.

**Option D:** Toxocara is a parasitic worm that can cause visceral larva migrans, a condition characterized by a range of systemic symptoms, but it is not typically associated with pleomorphic skin lesions.

**Clinical Pearl / High-Yield Fact**
When encountering a patient with a rash, remember that pleomorphism is a key feature of chickenpox. A diverse range of skin lesions, including different stages of development, is characteristic of this condition.

**Correct Answer:**
✓ Correct Answer: A. Chickenpox