A patient shows signs of a bullseye rash after hiking. What is the likely diagnosis?

The presence of a bullseye rash, also known as erythema migrans, is highly indicative of Lyme disease, particularly after a patient has been hiking or in areas where ticks are prevalent. This characteristic rash typically appears within days to weeks after exposure to a tick carrying Borrelia burgdorferi, the bacterium responsible for Lyme disease. While not all patients with Lyme disease will develop this rash, its appearance combined with the recent hiking activity strongly supports this diagnosis.

Rocky Mountain Spotted Fever does present with a rash, but it does not typically manifest as a bullseye and is characterized more by spotted or petechial rashes that appear after more acute symptoms. Chickenpox presents with a vesicular rash that progresses from macules to papules and then to vesicles, which is quite different from the bullseye pattern. Scabies causes itching and a rash, usually in specific areas, but it does not produce the distinctive bullseye rash seen in Lyme disease. Given these details, the likely diagnosis is classical Lyme disease, as reflected in the correct answer.