**Core Concept**
Measles is a highly contagious viral infection caused by the measles virus, which spreads through respiratory droplets. The infectious period is defined by the time when the virus is shed in the respiratory secretions, primarily during the pre- and post-rash phases.

**Why the Correct Answer is Right**
The measles virus is most infectious during the incubation phase, specifically **4 days before the rash appears** (when viremia is high and shedding begins), and continues for **5 days after the rash** (when the virus remains in the respiratory tract). This period is well-established in epidemiology and clinical practice. The virus is excreted in nasal secretions and saliva, and transmission occurs via droplet spread during this window.

**Why Each Wrong Option is Incorrect**
Option A: The period of one day before and four days after rash is too short and does not reflect the true infectious window. The virus is shed earlier and longer than this.
Option C: The incubation period (10–14 days) is not infectious; the virus is not shed during this time. Infection begins only after the pre-rash phase.
Option D: Scab falling occurs at the end of the disease and is not associated with viral shedding. The virus is not transmitted during this phase.

**Clinical Pearl / High-Yield Fact**
Always remember: **"4 days before, 5 days after" rash** — this is the standard infectious period for measles. This is critical for contact tracing and public health interventions.

✓ Correct Answer: B. Four days before and five days after rash