What is the differential diagnosis for a patient presenting with a flat, raised rash on their lower limbs to buttocks, considering potential underlying conditions such as skin allergies, diabetes, or poor circulation?

Differential Diagnosis for Flat Raised Rash on Lower Limbs to Buttocks

The most critical immediate consideration is Henoch-Schönlein purpura (IgA vasculitis), which classically presents with palpable purpura on the lower extremities and buttocks, particularly in children and young adults, and requires urgent evaluation for systemic involvement including renal disease.

Life-Threatening Conditions to Exclude First

Vasculitis and Systemic Disease

• Henoch-Schönlein purpura (IgA vasculitis) is the primary concern for a palpable purpuric rash distributed on lower extremities and buttocks, especially if accompanied by abdominal pain, arthralgia, or hematuria 1
• Rocky Mountain Spotted Fever must be excluded if fever is present, as it begins with blanching pink macules on ankles/wrists that progress to petechiae and spreads to palms/soles, carrying a 5-10% case-fatality rate 1
• If fever accompanies the rash, initiate doxycycline 100 mg twice daily immediately without waiting for laboratory confirmation, per CDC recommendations 1
• Obtain complete blood count, comprehensive metabolic panel, urinalysis (to assess for hematuria/proteinuria), and acute serology if fever is present 1

Critical Red Flags Requiring Immediate Action

• Fever + rash + headache + tick exposure or thrombocytopenia/hyponatremia mandates immediate doxycycline 1
• Do not exclude RMSF based on absence of tick bite history, as 40% of patients do not report tick exposure 1
• Do not wait for serologic confirmation before starting doxycycline if RMSF is suspected 1

Common Inflammatory Dermatoses

Contact Dermatitis (Allergic or Irritant)

• Allergic contact dermatitis presents as eczematous lesions that may be clinically indistinguishable from other eczematous conditions, with prevalence of 6-60% in atopic individuals 2
• The most common contact allergens include nickel, neomycin, fragrance, formaldehyde and other preservatives, lanolin, and rubber chemicals 2
• Patch testing is essential for diagnosis, applied to unaffected skin for 48 hours, with assessment at removal and again up to 7 days later for delayed reactions 2
• Consider patch testing when distribution is unusual or atypical (e.g., sides of feet), with later onset or new significant worsening, or with persistent/recalcitrant disease not responding to standard therapies 2
• Occupational dermatitis accounts for 70% of occupational skin disease, with hand involvement predominating, but lower extremity involvement can occur with specific exposures 1

Psoriasis

• Palmoplantar and lower extremity psoriasis presents with erythematous inflammatory plaques with silvery scale 1
• Personal or family history of psoriasis strongly favors this diagnosis, with plaques tending to be well-demarcated, indurated, and sharply defined 2
• Differential from chronic allergic contact dermatitis is difficult, especially with isolated lower extremity involvement 1
• Lower leg edema is common in extensive psoriasis and should not be confused with joint involvement 2

Atopic Dermatitis

• Atopic dermatitis presents with more intense pruritus and lichenification in chronic cases, often with personal or family history of atopy 2
• Distribution typically involves flexural areas more commonly than extensor surfaces 2
• Erythrodermic presentations may closely resemble other conditions including psoriasis and contact dermatitis 2

Infectious Causes

Viral Exanthems

• Enteroviral infections are the most common cause of maculopapular rashes, typically sparing palms, soles, face, and scalp 1
• Human herpesvirus 6 (roseola) presents with macular rash following high fever 1
• Parvovirus B19 causes "slapped cheek" appearance with possible truncal involvement 1

Bacterial and Fungal Infections

• Secondary bacterial infection may occur with crusting, weeping, or pustules, requiring treatment with appropriate antibiotics such as flucloxacillin for Staphylococcus aureus 3, 4
• Tinea infection is a common fungal condition in children affecting various body sites including lower extremities 5

Diagnostic Algorithm

Initial Assessment

• Determine if fever is present - if yes, immediately consider RMSF and initiate doxycycline while obtaining CBC, CMP, and acute serology 1
• Assess rash characteristics: color (erythematous vs purpuric), blanching vs non-blanching, presence of scale, distribution pattern 6
• Evaluate for systemic symptoms: abdominal pain, arthralgia, hematuria (suggests vasculitis), pruritus (suggests atopic/contact dermatitis) 6, 5

Key Clinical Features to Document

• Distribution pattern: symmetric vs asymmetric, involvement of buttocks and lower extremities specifically suggests HSP or contact dermatitis from clothing/footwear 1, 6
• Individual lesion characteristics: flat vs raised (palpable purpura suggests vasculitis), presence of scale (suggests psoriasis or dermatitis), umbilication (suggests molluscum) 6, 5
• Temporal evolution: rash after fever resolution (roseola), herald patch followed by Christmas tree pattern (pityriasis rosea), progressive spread (RMSF) 1, 5
• Associated symptoms: fever, pruritus, pain, joint symptoms, abdominal symptoms 6

Laboratory and Diagnostic Testing

• If vasculitis suspected: CBC, CMP, urinalysis, ESR/CRP, consider skin biopsy for immunofluorescence 1
• If contact dermatitis suspected: patch testing to unaffected skin, assessed at 48 hours and up to 7 days 2
• If infection suspected with fever: blood cultures, acute serology for rickettsial diseases 1
• Skin biopsy may aid diagnosis in atypical presentations, looking for atypical lymphocytes (cutaneous T-cell lymphoma), psoriasiform changes, or vasculitis 2

Treatment Approach Based on Diagnosis

For Contact Dermatitis

• Identify and avoid the suspected allergen through detailed history and patch testing 2
• Apply low-potency topical corticosteroids (hydrocortisone 1%) for limited periods, not exceeding 2-4 weeks 3
• Use mild, pH-neutral non-soap cleansers and fragrance-free emollients after bathing 3
• Avoid alcohol-containing preparations that worsen dryness 3

For Psoriasis

• Mid-potency topical corticosteroids (prednicarbate 0.02%) for significant erythema and inflammation 3
• Consider systemic therapy for extensive involvement (>10% BSA) 2
• Avoid long-term corticosteroid use due to risk of tachyphylaxis 3

For Suspected Vasculitis

• Urgent referral to rheumatology or dermatology for confirmation and systemic evaluation 1
• Monitor renal function closely with serial urinalysis 1
• Supportive care and systemic corticosteroids may be required for severe cases 1

Critical Pitfalls to Avoid

• Do not assume lower extremity rash is benign - multiple life-threatening infections and vasculitis present in this distribution 1
• Do not rely solely on clinical features to distinguish allergic from irritant contact dermatitis - patch testing is essential 2, 1
• Do not exclude RMSF based on absence of tick bite or lack of palmar/plantar involvement early in disease course 1
• Do not use potent topical corticosteroids in intertriginous areas due to increased absorption and risk of skin atrophy 4
• Do not overlook secondary bacterial infection - look for crusting, weeping, or grouped erosions suggesting herpes simplex 3, 4
• Do not confuse lower leg edema from extensive dermatitis with joint involvement 2

When to Refer to Specialist

• Diagnostic uncertainty or atypical presentation 3
• Failure to respond after 4 weeks of appropriate first-line therapy 3
• Suspected vasculitis requiring systemic evaluation 1
• Need for patch testing or second-line treatments 2, 3
• Recurrent severe flares despite optimal maintenance therapy 3