**Core Concept**
Measles is a highly contagious viral infection caused by the measles virus, leading to a range of complications, including respiratory, neurological, and ophthalmological issues. The measles virus affects the immune system, causing a significant increase in the production of pro-inflammatory cytokines, which can lead to tissue damage and organ dysfunction.

**Why the Correct Answer is Right**
The "least common" complication of measles is subacute sclerosing panencephalitis (SSPE), a rare but severe neurological disorder that occurs as a result of persistent measles virus infection. SSPE is characterized by a gradual degeneration of the brain, leading to cognitive impairment, seizures, and eventually, death. The measles virus integrates into the host's genome and remains dormant for years, causing a delayed onset of SSPE, typically 7-10 years after the initial infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** Pneumonia is a common complication of measles, accounting for approximately 1 in 5 measles-related deaths. It occurs due to the measles virus's immunosuppressive effects, making the individual more susceptible to secondary bacterial infections.
* **Option B:** Encephalitis is a well-documented complication of measles, occurring in approximately 1 in 1000 cases. It is characterized by inflammation of the brain, leading to symptoms such as seizures, confusion, and altered mental status.
* **Option C:** Corneal ulcers are a rare but serious complication of measles, occurring in approximately 1 in 100 cases. They are caused by the measles virus's immunosuppressive effects, making the individual more susceptible to secondary bacterial infections.

**Clinical Pearl / High-Yield Fact**
Measles is a highly contagious and vaccine-preventable disease. The measles vaccine is effective in preventing measles and its complications, including SSPE. It is essential to maintain high vaccination rates to prevent outbreaks and protect vulnerable populations, such as young children and individuals with compromised immune systems.

**Correct Answer:** B. Encephalitis.