**Core Concept**
Measles is a highly contagious viral infection caused by the measles virus, leading to a range of complications, particularly in children with compromised immune systems. The most common complications arise due to the virus's effect on the respiratory and central nervous systems.

**Why the Correct Answer is Right**
The most common complication of measles in children is pneumonia, which is responsible for approximately 70-90% of measles-related deaths worldwide. This is due to the measles virus's ability to infect the respiratory epithelium, leading to an inflammatory response that can cause the formation of hyaline membranes and subsequent respiratory failure. The measles virus also has a predilection for the lymphoid system, particularly the tonsils and adenoids, which can become infected and contribute to the development of pneumonia.

**Why Each Wrong Option is Incorrect**
**Option A:** Encephalitis is a serious complication of measles, but it is less common than pneumonia, occurring in approximately 1 in 1000 cases.
**Option B:** Measles can also cause otitis media, but it is not the most common complication in children.
**Option C:** Diarrhea is a common symptom of measles, but it is not typically considered a complication.

**Clinical Pearl / High-Yield Fact**
The measles vaccine is highly effective in preventing the disease and its complications, including pneumonia. It is recommended that children receive the first dose of the measles vaccine at 9-12 months of age, followed by a second dose at 4-6 years of age.

**Correct Answer: B. Otitis media**