**Core Concept**
Chicken pox, caused by the varicella-zoster virus, is a highly contagious infection that spreads through respiratory droplets, direct contact, or contact with contaminated surfaces. The infectivity period is characterized by the presence of the virus in the skin lesions and the respiratory tract.

**Why the Correct Answer is Right**
The correct answer is that chicken pox is highly contagious from 1-2 days before the onset of the rash to 4-5 days after the appearance of the rash. This period coincides with the viral shedding in the skin lesions and the respiratory tract. The varicella-zoster virus replicates in the skin cells and is released through the respiratory tract, allowing for efficient transmission to susceptible individuals.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because chicken pox is not highly contagious after the lesions have crusted over, which typically occurs 7-10 days after the onset of the rash.
* **Option B:** This option is incorrect because chicken pox is not highly contagious before the onset of the rash, although the patient may still be shedding the virus.
* **Option C:** This option is incorrect because chicken pox is not highly contagious in the prodromal phase, which typically occurs 1-2 days before the onset of the rash.

**Clinical Pearl / High-Yield Fact**
The infectivity period of chicken pox is often remembered using the mnemonic "1-2-4-5," which represents the days before and after the onset of the rash when the patient is contagious.

**Correct Answer: D. 1-2 days before the onset of the rash to 4-5 days after the appearance of the rash.**