Final answer:
Erythema nodosum is a condition causing red nodules on the skin due to inflammation in the subcutaneous fat cells, often triggered by streptococcal infections, autoimmune diseases, contact dermatitis, and hypersensitivity reactions.
Step-by-step explanation:
Erythema nodosum is a skin condition that manifests as inflammation in the subcutaneous fat cells of the hypodermis, leading to red nodules typically on the legs, especially the shins. The possible causes of erythema nodosum include infections such as those by Streptococcus pyogenes, which can breach the skin through cuts or abrasions, but it can also be triggered by a variety of other factors. These include autoimmune diseases like diabetes mellitus type I, multiple sclerosis, rheumatoid arthritis, contact dermatitis, and reactions to medications or substances, such as the tuberculin skin test or exposure to allergens like latex. It is important to note that erythema nodosum is non-suppurative, which means it does not involve pus formation like some other skin conditions caused by infections.