What type of lesions spread from the face and arms to the trunk and legs, all at the same stage?

The key characteristic of smallpox is that it presents with a specific progression of lesions. Unlike other viral infections, which may demonstrate different stages of lesions at different body sites, smallpox lesions appear synchronously at the same time. This means that when the rash develops, lesions occur simultaneously across various areas, including the face, arms, trunk, and legs.

In smallpox, after an incubation period, the disease manifests with a characteristic rash that evolves through distinct stages, such as macules, papules, vesicles, and ultimately pustules, which all progress together. This synchronous development is a hallmark feature that helps in the clinical identification of smallpox.

Other infections listed, such as chickenpox and shingles, follow different patterns. Chickenpox lesions start on the trunk and face and gradually progress to other parts of the body, often presenting in waves of different stages; shingles usually affects a specific dermatome and does not appear symmetrically or simultaneously across distant body areas. Measles presents with a rash that typically starts on the face and moves downwards, again not showing the same stage of development across the body simultaneously.

Therefore, the unique aspect of smallpox's synchronous lesion development across various regions supports its identification as the correct answer.