**Core Concept**
Measles is a highly contagious viral infection caused by the measles virus, leading to a range of complications due to the viral replication and immune response. The measles virus primarily targets the respiratory system, but it can also cause secondary infections and systemic complications.

**Why the Correct Answer is Right**
Subacute sclerosing panencephalitis (SSPE) is a rare but fatal complication of measles, occurring approximately 7-10 years after the initial measles infection. SSPE is caused by the persistence of the measles virus in the brain, leading to progressive neurological damage and inflammation. This condition is characterized by a gradual decline in cognitive function, seizures, and eventual death. The exact mechanisms underlying SSPE are not fully understood, but it is believed to result from the reactivation of latent measles virus in the brain.

**Why Each Wrong Option is Incorrect**
**Option A:** Diarrhea is a common complication of measles, especially in children, but it is not considered the least common complication. Diarrhea occurs due to the viral infection of the gastrointestinal tract and can lead to dehydration and electrolyte imbalances.
**Option B:** Pneumonia is a significant complication of measles, particularly in young children and immunocompromised individuals. Pneumonia occurs due to secondary bacterial or viral infections, which can be life-threatening if not treated promptly.
**Option C:** Otitis media is another common complication of measles, characterized by inflammation of the middle ear. Otitis media occurs due to the spread of the measles virus to the ear or secondary bacterial infections.

**Clinical Pearl / High-Yield Fact**
SSPE is a rare but significant complication of measles, highlighting the importance of measles vaccination in preventing this condition. The measles vaccine is a crucial tool in controlling measles outbreaks and reducing the risk of SSPE.

**✓ Correct Answer: D. SSPE**