**Question:** A 6-year-old boy develops symptoms of cough, fever, and malaise followed by a generalized maculopapular rash that has spread from the head downward. A clinical diagnosis of measles is made. A few days after the onset of the rash he is drowsy, lethargic, and complaining of headache. A lumbar puncture, electroencephalogram (EEG), and computerized tomography (CT) of the brain exclude other etiologies and confirm the diagnosis of measles virus encephalitis. Which of the following is the most likely delayed neurologic complication of measles virus encephalitis?

A. Encephalitis
B. Meningitis
C. Myelitis
D. Acute disseminated encephalomyelitis (ADEM)

**Correct Answer:** D. Acute disseminated encephalomyelitis (ADEM)

**Core Concept:** Measles virus encephalitis is a complication of measles infection that affects the brain and central nervous system (CNS). It is characterized by an acute, often self-limiting, inflammation of the brain and spinal cord. The measles virus is primarily known for causing respiratory and gastrointestinal symptoms but can also affect the CNS.

**Why the Correct Answer is Right:** Acute disseminated encephalomyelitis (ADEM) is a delayed neurologic complication of measles virus encephalitis. ADEM typically occurs weeks to months after the initial infection and is characterized by a widespread inflammation of the brain and spinal cord. While other options like encephalitis and meningitis are associated with measles virus encephalitis, they are acute events rather than delayed complications. Myelitis is inflammation of the spinal cord, which is a separate entity and not directly related to measles virus encephalitis.

**Why Other Options are Incorrect:**

A. Encephalitis: Measles virus encephalitis is an acute complication, not a delayed complication. ADEM is the most likely delayed neurologic complication of measles virus encephalitis.

B. Meningitis: While measles virus can cause meningitis, it is not the most likely delayed complication. ADEM is the more appropriate answer as it is a delayed neurologic complication.

C. Myelitis: Myelitis is inflammation of the spinal cord, which is a separate entity and not directly related to measles virus encephalitis. ADEM is the correct delayed neurologic complication.

**Clinical Pearls:**

Acute disseminated encephalomyelitis (ADEM) is a delayed neurologic complication of measles virus encephalitis, resulting from a delayed immune response to a previously established infection. It is characterized by widespread inflammation of the brain and spinal cord, which can lead to neurological deficits and seizures. Recognizing this delayed complication is crucial for appropriate management and prognosis.