**Core Concept**
Chicken pox and small pox are both caused by orthopoxviruses, but they can be distinguished by their clinical presentation. The key to differentiating between these two diseases lies in the morphology and distribution of the rash.

**Why the Correct Answer is Right**
The characteristic feature that helps differentiate chicken pox from small pox is the presence of **papules, vesicles, and crusts in various stages of development** in chicken pox. This is because the rash in chicken pox progresses rapidly, with new lesions appearing while others are still crusting over. In contrast, small pox typically presents with a more uniform rash, with lesions at the same stage of development.

* **Option A:** This option is incorrect because it does not accurately describe a characteristic of the rash in chicken pox.
* **Option B:** This option is incorrect because it is not a reliable method for differentiating between chicken pox and small pox.
* **Option C:** This option is incorrect because it is a characteristic of the rash in small pox, not chicken pox.

**Clinical Pearl / High-Yield Fact**
To remember the difference between chicken pox and small pox, use the mnemonic "C.P. = Crusts and Papules": Chicken pox is characterized by crusts and papules in various stages of development, while small pox is not.

**Correct Answer:** C. The presence of papules, vesicles, and crusts in various stages of development is characteristic of chicken pox.