A patient presents with well-demarcated erythema on his lower leg, raised and warm to touch, along with a fever. What is the likely diagnosis?

In this scenario, the presence of well-demarcated erythema, raised skin, warmth to touch, and fever strongly suggests erysipelas. Erysipelas is characterized by a clearly defined border between the affected and unaffected skin, which is a key feature that distinguishes it from cellulitis, where the borders are often less well-defined. The warmth and elevated temperature indicate an inflammatory response typically associated with an infection.

Erysipelas is primarily caused by group A Streptococcus and often presents with systemic symptoms such as fever, which is noted in this case. The raised appearance of the affected area is reflective of the dermal involvement typical of erysipelas.

Other potential diagnoses, such as cellulitis, impetigo, and dermatitis, do not fully align with the specific characteristics presented. Cellulitis usually has a more diffuse border, impetigo typically appears as crusted lesions, and dermatitis would not usually be associated with such clear demarcation and systemic symptoms like fever. Such details emphasize why erysipelas is the likely diagnosis in this patient.