**Core Concept:** Exanthema subitum is a sudden onset of widespread maculopapular or morbilliform rash, commonly seen in infants. It can be due to various conditions, including viral infections, drug reactions, and autoimmune reactions.

**Why the Correct Answer is Right:** Exanthema subitum in the context of the question refers to a sudden onset of rash in infants. The correct answer, **D. Viral infections**, is right because viral infections like herpes simplex virus, varicella-zoster virus, and parvovirus B19 can cause exanthema subitum in infants due to their immature immune system.

**Why Each Wrong Option is Incorrect:**

A. **N. Drug reactions**: While drug reactions can cause exanthema subitum, they are usually more gradual in onset and may involve specific drug-induced rashes.

B. **C. Autoimmune reactions**: Autoimmune reactions can lead to exanthema subitum, but they usually present with a more specific and localized rash rather than a sudden, widespread maculopapular rash seen in infants.

C. **Morbilli**: Morbilli refers to the measles virus, which is not the cause of exanthema subitum but rather a condition itself.

**Clinical Pearl / High-Yield Fact:** Exanthema subitum in infants is often referred to as "infantile exanthem" or "infantile morbilliform exanthem." It is important for clinicians to consider viral infections as the cause, as unnecessary investigations and treatments can be avoided by making an accurate clinical diagnosis.