**Core Concept**
The patient is presenting with fever, red eyes, and a rash, suggesting a diagnosis of measles. Measles is a highly contagious viral disease caused by the measles virus, which affects the respiratory system and the skin. The rash associated with measles is characterized by a maculopapular or morbilliform pattern.

**Why the Correct Answer is Right**
Measles can lead to several complications, particularly in adults and immunocompromised individuals. One of the most significant complications is **pneumonia**, which is caused by secondary bacterial infections such as Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus. The measles virus damages the ciliated epithelial cells in the respiratory tract, making it difficult for the body to clear out pathogens. This increases the risk of secondary bacterial infections, including pneumonia.

**Why Each Wrong Option is Incorrect**
**Option A:** Cerebellar ataxia is not a direct complication of measles, although it can occur due to post-infectious cerebellitis.
**Option B:** Stevens-Johnson syndrome (SJS) is a rare but severe skin and mucous membrane disorder that is not typically associated with measles.
**Option C:** Meningitis is a possible complication of measles, but it is not as common as pneumonia.

**Clinical Pearl / High-Yield Fact**
In adults, measles can lead to more severe complications than in children, including pneumonia, encephalitis, and death.

**Correct Answer:** C. Meningitis