What is the earliest clinical sign of radiation exposure?

Erythema, or skin reddening, is recognized as one of the earliest clinical signs of radiation exposure. This symptom arises from the inflammatory response initiated by radiation damaging the cells within the skin. When the skin is exposed to radiation, it can lead to localized damage where blood vessels become engorged, producing the characteristic redness associated with erythema.

On the other hand, leukopenia, which is a decrease in white blood cells, typically manifests later as radiation exposure continues to affect the bone marrow and the body’s ability to produce immune cells. Nausea can occur, particularly with higher levels of exposure, but it usually develops as part of the acute radiation syndrome rather than being an immediate indicator like erythema. Fever is also a response that can follow radiation exposure due to the body's reaction to injury and potential infection but is not an initial clinical sign.

Therefore, erythema stands out as the first observable change following exposure, making it a crucial sign for healthcare professionals to be aware of when assessing potential radiation damage.