**Question:** Most common complication of measles is:

A. Respiratory infection (bronchiolitis obliterans)
B. Encephalitis
C. Pneumonia
D. Dehydration

**Core Concept:** Measles is a highly contagious viral illness caused by the measles virus (family Paramyxoviridae, genus Morbillivirus). The disease typically presents with fever, cough, coryza, conjunctivitis, and a characteristic rash. The most common complications arise due to the virus's effects on the immune system and the severity of the initial illness.

**Why the Correct Answer is Right:** Encephalitis is the most common complication of measles, with a reported incidence ranging from 0.1% to 1%. Measles virus enters the central nervous system (CNS) through the respiratory tract and can cause encephalitis, which is characterized by inflammation of the brain tissue. This leads to neurological symptoms such as fever, seizures, and altered mental status.

**Why Each Wrong Option is Incorrect:**

A. **Respiratory infection (bronchiolitis obliterans):** While measles can cause respiratory complications like bronchopneumonia, bronchiolitis obliterans is a rare complication and not the most common complication of measles.

B. **Encephalitis (neurological complications):** While encephalitis can occur in severe cases of measles, it is not the most common complication.

C. **Pneumonia:** Pneumonia is a common complication of measles, but it is less likely than encephalitis as the most common complication.

D. **Dehydration:** Although dehydration can occur due to high fever, it is not the most common complication of measles.

**Clinical Pearl:** Measles complications can be severe and lead to significant morbidity and mortality. Early recognition and management of complications are crucial for patients with measles.

**Correct Answer:** Encephalitis (B) is the correct answer as it is the most common neurological complication of measles, occurring in up to 10% of cases. The virus can cause inflammation of the brain tissue, leading to neurological symptoms and complications.