**Core Concept**
Measles is a highly contagious viral infection caused by the measles virus, characterized by fever, cough, runny nose, and a characteristic rash. Complications of measles can be acute or subacute, and include respiratory, gastrointestinal, and central nervous system manifestations.

**Why the Correct Answer is Right**
Subacute sclerosing panencephalitis (SSPE) is a rare and serious complication of measles infection, occurring due to the persistence of the measles virus in the central nervous system. This leads to a progressive demyelinating encephalitis, characterized by inflammation and degeneration of brain tissue. SSPE typically occurs years after the initial measles infection, often in children and young adults, and has a poor prognosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Diarrhea is a common complication of measles, occurring due to viral gastroenteritis or secondary bacterial infections. It is a significant cause of morbidity and mortality in measles outbreaks, especially in malnourished children.
**Option B:** Pneumonia is a leading cause of death in measles cases, particularly in children under the age of 5. It can be caused by secondary bacterial infections, such as Streptococcus pneumoniae or Haemophilus influenzae, and requires prompt antibiotic treatment.
**Option C:** Otitis media, or middle ear infection, is a frequent complication of measles, occurring due to Eustachian tube dysfunction and secondary bacterial infections. It can lead to hearing loss and other complications if left untreated.

**Clinical Pearl / High-Yield Fact**
Measles complications can be prevented through timely administration of antibiotics, antiviral therapy, and supportive care. SSPE, however, is a rare but devastating complication that highlights the importance of measles vaccination in preventing long-term neurological consequences.

✓ Correct Answer: D. SSPE