Eritrasma

01Pathophysiology

Erythrasma is a superficial bacterial infection of the skin caused by Corynebacterium minutissimum, a gram-positive, catalase-positive, non-spore-forming bacillus that is part of the normal cutaneous flora. The organism proliferates within the uppermost layers of the stratum corneum without invading deeper tissue. This is why erythrasma is a surface-level process with no systemic signs such as fever, lymphadenopathy, or leukocytosis.

The bacterium thrives in warm, moist environments. This explains why the infection localizes to intertriginous areas (groin, axillae, interdigital web spaces, inframammary folds), where skin-on-skin contact traps moisture and heat. Conditions that increase skin surface moisture or compromise local host defenses, particularly diabetes mellitus and obesity, predispose to overgrowth. Diabetic patients have altered skin pH, increased glucose in sweat, and impaired immune surveillance, all of which create an ideal environment for C. minutissimum.

A critical concept for exam purposes is the organism's production of coproporphyrin III, a porphyrin pigment. This pigment is the direct reason for the hallmark coral-red fluorescence under Wood's lamp examination. The connection between pathophysiology and the diagnostic finding is direct: the bacterium's metabolic byproduct is the diagnostic marker.

Because the infection is confined to the dead keratin layer, the inflammatory response is minimal. This explains the lack of vesicles, pustules, or a raised active border, which is a key clinical differentiator from dermatophyte infections (tinea).

02Classification and Clinical Manifestation

Erythrasma does not have a formal staging system, but it is clinically classified by its location and extent.

Clinical Pearl: The intertriginous form is the most commonly tested. Vignettes typically describe a diabetic or obese patient with a chronic, mildly itchy, brownish patch in the groin that "hasn't responded to antifungals." That last detail is the strongest clinical clue.

03Diagnostic Workup

Wood's Lamp Examination is the cornerstone of diagnosis and the answer the exam is almost always looking for. The lamp emits ultraviolet light at approximately 365 nm, causing the coproporphyrin III produced by C. minutissimum to emit a characteristic coral-red (pinkish-red) fluorescence. This test is rapid, non-invasive, and highly characteristic.

One important caveat: the fluorescence can be washed away if the patient recently bathed or applied topical agents to the area. If the vignette mentions a patient who cleaned the area before the visit, this can produce a false-negative Wood's lamp result. This is a testable detail.

KOH preparation is not the diagnostic test for erythrasma, but it plays an important role in the workup. Because the clinical appearance overlaps heavily with tinea cruris and tinea pedis, a negative KOH in the presence of coral-red fluorescence strongly confirms erythrasma and rules out dermatophyte infection. Think of KOH as the test that eliminates the main differential.

Culture of C. minutissimum is technically the most accurate test, but it is slow, expensive, and almost never required in practice. For exam purposes, the answer to "What test confirms the diagnosis?" is Wood's lamp, not culture, because the coral-red fluorescence pattern is considered pathognomonic in clinical context.

04Management and Treatment

For localized erythrasma, topical therapy is sufficient and is the first-line approach. Erythromycin 2% cream or solution applied twice daily for 14 days is the standard treatment. Topical clindamycin 1% is equally effective and is a reasonable alternative. The choice between these two is not typically tested; what matters is knowing that localized disease is treated topically with a macrolide or lincosamide.

For extensive, recurrent, or refractory disease, oral erythromycin 250 mg four times daily for 14 days is the treatment of choice. An increasingly cited alternative is a single 1 g dose of oral clarithromycin, which has shown high cure rates and excellent compliance. On exams, if the vignette describes widespread disease or failure of topical therapy, the answer shifts to systemic macrolide therapy.

Adjunctive measures are important for preventing recurrence. The organism thrives in moist environments, so patients should be counseled on keeping intertriginous areas dry, using absorbent powders, wearing loose-fitting clothing, and managing underlying conditions such as diabetes and obesity. Benzoyl peroxide wash or antiseptic cleansers can reduce bacterial load and lower recurrence rates.

Contraindications and Considerations: Macrolides (erythromycin, clarithromycin) carry a risk of QT prolongation and should be used with caution in patients on other QT-prolonging medications. Clarithromycin is contraindicated in pregnancy. If these agents cannot be used, topical therapy with clindamycin or fusidic acid is a safe alternative.

The "Next Best Step" logic for a typical vignette: patient with a chronic reddish-brown groin rash that failed topical antifungals, Wood's lamp shows coral-red fluorescence. The next best step is topical erythromycin 2% (if localized) or oral erythromycin (if widespread).

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students miss erythrasma is by defaulting to a fungal diagnosis when they see a reddish-brown, scaly, intertriginous rash. The groin location, the scaling, and the association with diabetes all scream "tinea cruris," and many students pick an antifungal without considering a bacterial etiology. The exam exploits this tendency by including a detail that should redirect your thinking: either a negative KOH prep, a history of failed antifungal therapy, or an outright mention of Wood's lamp with coral-red fluorescence.

Another common trap involves confusing fluorescence colors. Students must commit to memory: coral-red = erythrasma (C. minutissimum), yellow-green = tinea versicolor (Malassezia), and bright green = Pseudomonas. The fluorescence color is the discriminator, and substituting one for another will cost you the question.

A subtler trap is the interdigital form. When a vignette describes scaling and maceration of the toe web spaces, students jump to tinea pedis. The clue is again a negative KOH or a mention of fluorescence. Recognize that erythrasma and dermatophyte infections can coexist in the same web space, so a positive KOH does not automatically rule out concurrent erythrasma.

Finally, remember that the core competency being tested here is pattern recognition of a benign, superficial bacterial skin infection that mimics a fungal process. The exam wants to confirm that you can use Wood's lamp findings and KOH results together to distinguish bacterial from fungal etiologies in intertriginous dermatoses and choose the correct antimicrobial class (antibacterial, not antifungal) for treatment.