Erythrasma — Viva

Q1: Definition and clinical presentation (2 min)

What is erythrasma? Name the causative organism and describe its typical clinical presentation and distribution.

Model answer: Erythrasma is a chronic, superficial bacterial infection of the intertriginous skin caused by Corynebacterium minutissimum, a Gram-positive diphtheroid (some taxonomic schemes place it in Kocuria). It presents as well-demarcated, reddish-brown, finely scaling macules and patches in the groin, axillae, toe webs, submammary folds and gluteal cleft. The 4th interdigital toe web is the most common single site. Lesions are often asymptomatic or mildly pruritic. [1]

Q2: Pathophysiology and diagnosis (2 min)

What is the pathognomonic diagnostic sign of erythrasma? Explain the underlying mechanism.

Model answer: The pathognomonic sign is coral-red (salmon-pink) fluorescence under Wood's lamp (UV-A, 365 nm). This is caused by coproporphyrin III, a water-soluble porphyrin produced by Corynebacterium minutissimum in the stratum corneum. The organism remains superficial, produces proteolytic enzymes that cause fine scale, and does not invade living tissue. [1]

Q3: Differential diagnosis (3 min)

What conditions would you consider in the differential diagnosis of erythrasma, and how would you distinguish them?

Model answer: Differential diagnoses include tinea cruris, candidal intertrigo, inverse psoriasis, irritant intertrigo, seborrhoeic dermatitis and contact dermatitis. Tinea cruris is more pruritic, has an active scaly border, is KOH positive and Wood's lamp negative. Candidal intertrigo is bright red with satellite pustules, KOH positive and shows no fluorescence. Inverse psoriasis presents as salmon-pink, smooth plaques with minimal scale and no fluorescence. Wood's lamp is the key bedside discriminator: coral-red fluorescence is pathognomonic for erythrasma. [1]

Q4: Investigations (2 min)

What investigations are useful in erythrasma? When is each indicated?

Model answer: Wood's lamp examination is the diagnostic test of choice. Skin scraping with KOH microscopy is used to exclude tinea and candidiasis when the diagnosis is uncertain. Gram stain and culture are rarely needed but may be useful in atypical, treatment-resistant or immunocompromised cases. In extensive or recurrent disease, screen for diabetes mellitus (fasting glucose, HbA1c) and consider HIV testing or other causes of immunosuppression. [1]

Q5: Management (3 min)

Outline your stepwise approach to management of erythrasma. Include first-line, second-line and preventive measures.

Model answer: For localised disease, use topical therapy for 2–4 weeks: clotrimazole or miconazole (imidazoles with antibacterial activity), or topical antibacterials such as fusidic acid, erythromycin, clindamycin or mupirocin. For extensive, recurrent or refractory disease, use oral macrolides: erythromycin 500 mg four times daily for 7–14 days, or clarithromycin 500 mg twice daily for 7 days. Tetracycline is an alternative in adults but is contraindicated in pregnancy and children. Preventive measures include keeping folds dry, using drying powders, wearing breathable cotton clothing, weight reduction, diabetes control, treating hyperhidrosis and treating coexisting fungal infections. [1]

Q6: Complications and prognosis (2 min)

What are the main complications and pitfalls of erythrasma? What is the prognosis?

Model answer: Complications are mainly diagnostic (misdiagnosis as tinea or candidiasis) and recurrent (if predisposing factors are not addressed). Secondary bacterial infection (cellulitis, erysipelas) can occur in macerated folds. Rarely, systemic C. minutissimum infection is reported in immunocompromised hosts. The prognosis is excellent with appropriate treatment; lesions usually clear within 2–4 weeks, but recurrence is common unless the moist, occlusive environment is corrected. [1]

Q7: Special populations (2 min)

How does your management change in pregnancy, children and immunocompromised patients?

Model answer: In pregnancy, topical imidazoles or antibacterials are first-line; oral macrolides are generally acceptable if needed, but tetracycline is contraindicated. In children, topical therapy is preferred; oral macrolides require weight-based dosing, and tetracycline is contraindicated under 8 years of age. In immunocompromised patients, consider culture, have a lower threshold for systemic therapy, and be alert to co-infections and rare systemic disease. [1]

Q8: Exam minutiae (2 min)

Why are imidazole antifungals effective for a bacterial infection? Which underlying condition should you screen for in extensive or recurrent disease?

Model answer: Imidazoles such as clotrimazole and miconazole have antibacterial activity against C. minutissimum because they affect membrane synthesis pathways in this organism. Underlying diabetes mellitus should be screened for in extensive, recurrent or treatment-resistant erythrasma with fasting glucose and HbA1c. [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