**Core Concept**
Post-exposure measles vaccination is a strategy to prevent measles in individuals who have been exposed to the virus. This approach relies on the immune system's ability to mount a response to the vaccine before the onset of the disease. The window for administering the vaccine post-exposure is crucial for its effectiveness.

**Why the Correct Answer is Right**
Measles vaccine contains a live, attenuated virus that stimulates the body's immune system to produce antibodies and immune cells that can recognize and respond to the wild-type virus. When administered within a specific time frame after exposure, the vaccine can induce immunity before the onset of the disease, thereby preventing measles. The optimal window for post-exposure measles vaccination is generally considered to be within 72 hours of exposure.

**Why Each Wrong Option is Incorrect**
**Option A:** There is no widely accepted evidence to support administering measles vaccine beyond 96 hours post-exposure.
**Option B:** The World Health Organization recommends administering measles vaccine within 72 hours of exposure, not 48 hours.
**Option C:** The Centers for Disease Control and Prevention (CDC) do not recommend administering measles vaccine beyond 72 hours post-exposure.

**Clinical Pearl / High-Yield Fact**
When considering post-exposure measles vaccination, it is essential to remember that the vaccine is most effective when administered within 72 hours of exposure, as the risk of developing measles decreases significantly after this time frame.

**Correct Answer: B. 72 days**