What are the possible causes of erythematous rashes in the axilla, neck, inguinal area, umbilical area, and flank area in an adult patient with no significant medical history?

Erythematous Rashes in Intertriginous and Flexural Areas

The most likely diagnoses for erythematous rashes affecting the axilla, neck, inguinal area, umbilical area, and flank are intertrigo (fungal or bacterial), inverse psoriasis, or cutaneous manifestations of systemic histiocytic disorders, with the distribution pattern strongly suggesting intertriginous involvement requiring immediate exclusion of life-threatening conditions before pursuing common dermatologic causes. 1

Immediate Life-Threatening Exclusions

Before considering benign causes, you must actively exclude Rocky Mountain Spotted Fever (RMSF), particularly if the patient has fever, headache, or myalgia:

• Initiate empiric doxycycline 2.2 mg/kg orally twice daily immediately if any clinical suspicion exists, without waiting for serologic confirmation. 1
• Up to 40% of RMSF patients report no tick bite history, and ticks preferentially attach in the axillae and inguinal regions—the exact distribution described in this case. 1
• RMSF causes death within 9 days if untreated, with mortality up to 20% without prompt treatment. 1
• Query recent outdoor activities, tick exposure, and travel to endemic areas immediately. 1

If fever >38°C is present with the rash, consider meningococcemia and administer concurrent ceftriaxone if this cannot be excluded. 1

Common Intertriginous Dermatologic Causes

Fungal Intertrigo (Candidiasis/Dermatophytosis)

• Erythematous patches with satellite lesions in skin folds (axillae, groin, umbilical area) are classic for candidal intertrigo. 2
• Apply topical antifungals to affected areas, keep skin folds completely dry, and ensure daily clothing changes. 1
• Use low-potency hydrocortisone 1% for intertriginous areas if inflammation is severe, but never use high-potency steroids in axillae or groin due to increased absorption and atrophy risk. 1

Inverse Psoriasis

• Smooth, well-demarcated erythematous plaques in flexural areas without typical scaling (due to moisture in folds).
• Oral antihistamines for pruritus if present. 1

Drug-Induced Eczematous Dermatitis

• Erythema and xerosis especially in skin folds, with neck and axillary creases affected in over 90% of cases. 1
• If >50% body surface involvement (Grade 3) or suspicion of DRESS/Stevens-Johnson Syndrome exists, immediately discontinue all potentially causative medications and arrange emergency hospitalization. 1

Systemic Histiocytic Disorders (Rare but Important)

Erdheim-Chester Disease (ECD)

• Yellow-brown xanthomatous papulonodules in axillary and inguinal folds affecting 20-30% of patients. 3
• May present as initial manifestation with yellowish-brown papules or plaques on face, neck, axilla, trunk, or groin. 3
• Requires tissue biopsy for definitive diagnosis and comprehensive systemic evaluation including cardiac imaging (50-70% have cardiovascular involvement) and pulmonary assessment. 1
• Look for "coated aorta" on CT scan (circumferential soft tissue sheathing) and "hairy kidney" (perinephric infiltration). 3

Langerhans Cell Histiocytosis (LCH)

• Erythematous papular rash in groin, abdomen, chest, or back. 1
• Tissue biopsy required for definitive diagnosis. 1

Rosai-Dorfman Disease (RDD)

• Red-to-brown macules or papules in 50% of patients, can manifest as panniculitis with tender subcutaneous nodules. 1

Diagnostic Algorithm

Step 1: Assess for systemic symptoms (fever, headache, myalgia, muscle weakness):

• If present → Start doxycycline immediately for presumed RMSF and obtain convalescent serology 2-4 weeks later. 1
• If muscle weakness with photosensitive rash → Consider dermatomyositis. 1

Step 2: Characterize the rash morphology:

• Petechial/purpuric → Emergency evaluation for meningococcemia or vasculitis. 1
• Erythematous with scaling borders → Fungal infection (tinea). 1, 2
• Erythematous without scaling in moist folds → Candidal intertrigo or inverse psoriasis. 1
• Yellow-brown papulonodules → Histiocytic disorder requiring biopsy. 3, 1

Step 3: If no systemic symptoms and typical intertriginous pattern:

• Trial of topical antifungal with low-potency hydrocortisone. 1
• If no improvement in 2 weeks → Obtain skin biopsy. 1

Critical Pitfalls to Avoid

• Never delay doxycycline if RMSF is suspected—mortality increases dramatically with delayed treatment. 1
• Never assume absence of tick bite excludes RMSF, as 40% of patients have no bite history. 1
• Never use high-potency corticosteroids in axillae or groin due to increased absorption and atrophy risk. 1
• Never dismiss xanthomatous lesions in flexural areas without systemic workup, as ECD has 50-70% cardiovascular involvement that can be life-threatening. 3, 1