What observation suggests that a patient has developed an infection due to skin anthrax?

The observation indicating that a patient has developed an infection due to skin anthrax is the presence of a visible rash and lesions on the skin. Skin anthrax, or cutaneous anthrax, typically starts as a small bump that resembles an insect bite and can progress into a sore with a characteristic black necrotic center, often accompanied by swelling and drainage. The visible rash and lesions are key indicators of the infection as they are directly linked to the entry of the Bacillus anthracis spores through cuts or abrasions in the skin, leading to localized infection.

Other factors, such as swelling and redness around a burn, while indicative of an infection, are less specific and could result from a variety of skin issues not limited to anthrax. Symptoms like rapid heartbeat and nausea may indicate systemic involvement or other unrelated conditions, and fever and muscle pain could point towards various infections but are not specific to skin anthrax. The unique presentation of the rash and lesions is what makes it a definitive sign of cutaneous anthrax.