Erythema nodosum — SAQ

Stem

A 28-year-old woman presents with a five-day history of bilateral, tender, red nodules on her shins, a low-grade fever and painful ankles. She has no significant past medical history and takes the oral contraceptive pill. She had a sore throat two weeks ago that settled without treatment. This presentation is typical of erythema nodosum, the commonest septal panniculitis.[1]

Questions

a) What is the most likely diagnosis and what two clinical features support it? (2 marks)[1]

b) Outline a focused investigation plan to identify the underlying cause, naming the core panel and one targeted test for each likely trigger in this patient. (3 marks)[1]

c) Describe your stepwise management, including a drug with dose, route and rationale. (3 marks)[1][7]

d) What single diagnosis must you exclude before prescribing a corticosteroid, and why? What safety-net advice will you give the patient? (2 marks)[1]

Model answer

a) Diagnosis: erythema nodosum (acute septal panniculitis). Supporting features: bilateral, symmetric, tender erythematous subcutaneous nodules on the anterior shins with bruise-like colour evolution, plus arthralgia and a recent sore throat (streptococcal trigger). It resolves without ulceration or scarring.[1][3]

b) Core panel (every patient): chest X-ray (sarcoidosis, TB, fungal), ASO titre and throat swab (streptococcal — raised given the recent pharyngitis), drug history (OCP), pregnancy test, CBC with ESR and CRP. Targeted: in this patient streptococcal pharyngitis is the leading cause (ASO, throat swab); the OCP is a drug trigger to review; if any GI symptoms, add stool calprotectin and consider colonoscopy for IBD. Biopsy is NOT required when the pattern is classic.[1]

c) Stepwise management: (1) treat the cause — penicillin V 500 mg orally four times daily for 10 days for confirmed strep, and review the OCP; (2) bed rest, leg elevation, cool compresses; (3) ibuprofen 400 mg orally three times daily with food for 1-2 weeks (inhibits cyclo-oxygenase, reduces prostaglandin-mediated pain and inflammation); (4) potassium iodide 300 to 900 mg per day for refractory disease.[1] Most cases resolve over 2-6 weeks; potassium iodide suppresses neutrophil chemotaxis but is contraindicated in pregnancy and requires TSH monitoring.[7]

d) Exclude active tuberculosis (IGRA or Mantoux plus chest X-ray) before any corticosteroid — steroids can activate or disseminate latent TB.[1] Safety-net: return urgently if nodules ulcerate, spread beyond the shins, persist beyond six weeks, or if new symptoms (fever, weight loss, cough, diarrhoea) appear.

References

1. [1]Pérez-Garza DM, Chavez-Alvarez S, Ocampo-Candiani J, et al. Erythema Nodosum: A Practical Approach and Diagnostic Algorithm. American Journal of Clinical Dermatology, 2021.PMID 33683567
2. [3]Wick MR. Panniculitis: A summary. Seminars in diagnostic pathology, 2017.PMID 28129926
3. [7]Goel N, Doshi BR Potassium Iodide in Dermatology - Recent Advances in Mechanism of Action, Preparation, Uses and Adverse Effects. Indian journal of dermatology, 2025.PMID 40487487