You are a helpful, respectful, and honest assistant. Always answer as helpfully as possible, while being safe. Your answers should not include any harmful, unethical, racist, sexist, toxic, dangerous, or illegal content. Please ensure that your responses are socially unbiased and positive in nature. If a question does not make any sense or is not factually coherent, explain why instead of answering something not correct. If you don't know the answer to a question, please don't share false information.
What is Erythema nodosum and explain in detail?
Erythema nodosum (EN) is an inflammatory condition characterized by inflammation of the fat cells under the skin, resulting in tender red nodules or lumps that are usually seen on both shins. It can be caused by a variety of conditions, and typically resolves spontaneously within 30 days. It is common in young people aged 12–20 years.

Signs and symptoms
Pre-eruptive phase
The first signs of erythema nodosum are often flu-like symptoms such as a fever, cough, malaise, and aching joints. Some people also experience stiffness or swelling in the joints and weight loss.

Eruptive stage
Erythema nodosum is characterised by 1–2-inch (25–51 mm) nodules (rounded lumps) below the skin surface, usually on the shins. These subcutaneous nodules can appear anywhere on the body, but the most common sites are the shins, arms, thighs, and torso. Each nodule typically disappears after around two weeks, though new ones may continue to form for up to six or eight weeks. A new nodule usually appears red and is hot and firm to the touch. The redness starts to fade and it gradually becomes softer and smaller until it disappears. Each nodule usually heals completely without scarring over the course of about two weeks. Joint pain and inflammation sometimes continue for several weeks or months after the nodules appear.Less common variants of erythema nodosum include:

Ulcerating forms, seen in Crohns disease
Erythema contusiforme,  when a subcutaneous hemorrhage (bleeding under the skin) occurs with an erythema nodosum lesion, causing the lesion to look like a contusion (bruise)
Erythema nodosum migrans (also known as subacute nodular migratory panniculitis), a rare form of chronic erythema nodosum characterized by asymmetrical nodules that are mildly tender and migrate over time.

Causes
EN is associated with a wide variety of conditions.

Idiopathic
About 30–50% of EN cases are idiopathic (of an unknown cause).

Infection
Infections associated with EN include:
Streptococcal infection which, in children, is by far the most common precipitant
Primary infection of tuberculosis
Mycoplasma pneumoniae
Histoplasma capsulatum
Yersinia
Lymphogranuloma venereum (LGV), caused by the bacteria Chlamydia trachomatis
Epstein-Barr virus
Coccidioides immitis (Valley fever)
Cat scratch disease

Autoimmune disorders
Autoimmune disorders associated with EN include:
Inflammatory bowel disease (IBD): about 15% of patients develop erythema nodosum.
Behçets disease
Sarcoidosis

Pregnancy
Pregnancy may be associated with EN.

Medications
Medications associated with EN include:
Omeprazole
Sulfonamides
Penicillins
Bromides
Hepatitis B vaccination

Cancer
Cancers associated with EN include:
Non-Hodgkins lymphoma (NHL)
Carcinoid tumours
Pancreatic cancerEN may also be due to excessive antibody production in lepromatous leprosy leading to deposition of immune complexes.There is an association with the HLA-B27 histocompatibility antigen, which is present in 65% of patients with erythema nodosum.A useful mnemonic for causes is SORE SHINS (Streptococci, OCP, Rickettsia, Eponymous (Behçet), Sulfonamides, Hansens Disease (Leprosy), IBD, NHL, Sarcoidosis.

Pathophysiology
Erythema nodosum is probably a delayed hypersensitivity reaction to a variety of antigens. Although circulating immune complexes have been demonstrated in patients with inflammatory bowel disease, they have not been found in idiopathic or uncomplicated cases.

Diagnosis
Erythema nodosum is diagnosed clinically. A biopsy can be taken and examined microscopically to confirm an uncertain diagnosis. Microscopic examination usually reveals a neutrophilic infiltrate surrounding capillaries that results in septal thickening, with fibrotic changes in the fat around blood vessels. A characteristic microscopic finding is radial granulomas, well-defined nodular aggregates of histiocytes surrounding a stellate cleft.Additional evaluation should be performed to determine the underlying cause of erythema nodosum. This may include a full blood count, erythrocyte sedimentation rate (ESR), antistreptolysin-O (ASO) titer and throat culture, urinalysis, intradermal tuberculin test, and a chest x-ray. The ESR is typically high, the C-reactive protein elevated, and the blood showing an increase in white blood cells.The ESR is initially very high and falls as the nodules of erythema nodosum. The ASO titer is high in cases associated with a streptococcal throat infection