Erythrasma — SAQ

Stem

A 48-year-old man presents with a 3-month history of well-demarcated, reddish-brown, finely scaling patches in both groin folds and the 4th interdigital toe webs. The rash is mildly itchy but not painful. On examination the lesions have sharp borders and no satellite pustules. Wood's lamp examination reveals coral-red fluorescence in the affected areas.

Questions

a) What is the most likely diagnosis, and what is the causative organism? (2 marks) [1]

b) What investigations would you perform to confirm the diagnosis and exclude differentials? (3 marks) [1]

c) Outline your stepwise management plan for this patient. (3 marks) [1]

d) What predisposing factors should be assessed, and what is the prognosis? (2 marks) [1]

Model answer

a) Diagnosis and causative organism (2 marks)

• Most likely diagnosis: erythrasma (1 mark).
• Causative organism: Corynebacterium minutissimum, a Gram-positive diphtheroid; some taxonomic schemes place it in Kocuria (1 mark). [1]

b) Investigations (3 marks)

• Wood's lamp examination (UV-A 365 nm): pathognomonic coral-red fluorescence from coproporphyrin III (1 mark).
• Skin scraping with KOH microscopy: to exclude tinea cruris (hyphae) and candidal intertrigo (pseudohyphae and budding yeast); negative in erythrasma (1 mark).
• Screen for predisposing disease: fasting plasma glucose and HbA1c for diabetes mellitus; consider HIV and obesity assessment if extensive or recurrent (1 mark). [1]

c) Management plan (3 marks)

• Topical therapy for localised disease: clotrimazole 1% or miconazole 2% cream twice daily for 2–4 weeks, OR topical antibacterial such as fusidic acid 2%, erythromycin 2%, clindamycin 1% or mupirocin 2% twice daily (1 mark).
• Extensive disease (multiple sites, or failure of topical therapy): oral erythromycin 500 mg four times daily for 7–14 days OR clarithromycin 500 mg twice daily for 7 days (1 mark).
• Adjunctive measures: keep folds dry, use drying powders, wear breathable cotton clothing, treat hyperhidrosis, manage obesity and diabetes, and treat coexisting tinea pedis or candidal intertrigo (1 mark). [1]

d) Predisposing factors and prognosis (2 marks)

• Predisposing factors: warm humid climate, hyperhidrosis, obesity, diabetes mellitus, immunosuppression, occlusive clothing/footwear (1 mark).
• Prognosis: excellent with appropriate treatment; recurrence is common if predisposing factors are not addressed; no scarring because infection is confined to the stratum corneum (1 mark). [1]

References

1. [1]Forouzan P, Cohen PR. Erythrasma Revisited: Diagnosis, Differential Diagnoses, and Comprehensive Review of Treatment Cureus, 2020.PMID 33145138
2. [2]Radhakrishnan S, Logamoorthy R, Karthikeyan K, et al. Erythrasma: a systematic review of interventions Clin Exp Dermatol, 2025.PMID 40635638
3. [3]Holdiness MR. Management of cutaneous erythrasma Drugs, 2002.PMID 12010076
4. [4]Blasco-Morente G, Arias-Santiago S, Pérez-López I, et al. Coral-Red Fluorescence of Erythrasma Plaque Sultan Qaboos Univ Med J, 2016.PMID 27606125
5. [5]Sebaratnam DF, Lee S. <em>Corynebacterium minutissimum</em> infection: erythrasma Med J Aust, 2017.PMID 28987127
6. [6]Robles-Silva C, Hidalgo L, Vera-Kellet C, et al. Erythrasma and the role of Wood's light: A simple diagnosis pearl Med Clin (Barc), 2023.PMID 37517932
7. [7]Garcia-Souto F, et al. Visual Dermatology: Erythrasma Fluorescence Under Wood's Lamp J Cutan Med Surg, 2020.PMID 31994922
8. [8]Hamann K, Thorn P. Systemic or local treatment of erythrasma? A comparison between erythromycin tablets and Fucidin cream in general practice Scand J Prim Health Care, 1991.PMID 2041927
9. [9]Sarkany I, Taplin D, Blank H. The etiology and treatment of erythrasma J Invest Dermatol, 1961.PMID 14497222