Final answer:
Erysipelas is a raised, intense rash with clear borders often on the face or legs, while cellulitis is a diffuse, painful, red rash without clear edges. Penicillin is commonly used to treat both, but erythema nodosum, presenting with red nodules, usually does not require antibiotics and is self-limiting.
Step-by-step explanation:
To differentiate between erysipelas and cellulitis, it is important to look for specific clinical signs. Erysipelas is characterized by an intense, raised rash with clear borders, typically caused by Streptococcus pyogenes and often found on the legs or face. The affected area is usually very red and may be accompanied by systemic symptoms such as fever and chills. In contrast, cellulitis appears as a diffuse, painful, red rash without raised borders, affecting the dermis and hypodermis. It can be caused by both S. pyogenes and staphylococci, and the infection typically spreads without clear margins.
Treatment for both conditions often involves antibiotics, with penicillin being commonly prescribed due to the fact that resistance has not been widespread in streptococci. Immediate medical attention is necessary when symptoms progress rapidly, as this could indicate a more serious infection such as necrotizing fasciitis. For erythema nodosum, which is a self-resolving condition presenting with red nodules on the shins, the treatment includes NSAIDs, cool compresses, elevation, and bed rest, rather than antimicrobial drugs.