What is the differential diagnosis for an adult patient with a petechial rash in the inguinal area, pain below the rash, and pain in the left arm below the elbow, potentially with underlying vascular or hematological conditions?

Differential Diagnosis for Inguinal Petechial Rash with Regional and Arm Pain

The most critical immediate consideration is Rocky Mountain Spotted Fever (RMSF), which requires empiric doxycycline therapy without delay, as 50% of deaths occur within 9 days of illness onset. 1

Life-Threatening Causes Requiring Immediate Action

Rickettsial Infections

• RMSF presents with petechiae that begin as small blanching pink macules on ankles, wrists, or forearms by day 5-6, evolving to maculopapular lesions with central petechiae, though up to 20% lack rash entirely. 1 The inguinal location is atypical but does not exclude this diagnosis.
• Start doxycycline 100 mg twice daily immediately if RMSF cannot be excluded, even without confirmed tick exposure (present in only 60% of cases). 1, 2
• Associated symptoms include fever exceeding 39°C, severe headache, and myalgias. 1

Meningococcemia

• Neisseria meningitidis causes petechial or purpuric rash that can rapidly progress to purpura fulminans, typically with high fever, severe headache, and altered mental status. 1
• Add ceftriaxone empirically if meningococcemia cannot be excluded based on clinical presentation. 1
• Progresses more rapidly than RMSF. 1

Fat Embolism Syndrome

• The classical triad includes altered mental status, respiratory distress, and petechial rash occurring 12-36 hours after long-bone or pelvic fractures. 3
• Consider if recent trauma, orthopedic surgery (knee/hip prostheses, endomedullary nailing), or sickle cell disease. 3
• The inguinal petechiae combined with arm pain below the elbow could represent this pattern if there was recent trauma or orthopedic intervention. 3

Bacterial Endocarditis

• Can cause petechiae in patients with cardiac risk factors (prosthetic valves, IV drug use, indwelling catheters). 1
• Septic emboli may explain both the petechial rash and limb pain if embolic phenomena are present. 3

Vascular and Embolic Causes

Cutaneous Leukocytoclastic Vasculitis

• Presents as diffuse petechial rash and non-blanching palpable purpura, particularly on lower extremities, which can coalesce, blister, and ulcerate. 4
• Can be drug-induced (anticoagulants like apixaban/rivaroxaban) with negative ANCA titers. 4
• Requires skin biopsy for definitive diagnosis showing perivascular inflammation. 3, 4

Adult-Onset Still's Disease (AOSD)

• Can present with vasculitic purpuric rash associated with mixed cryoglobulinemia. 3, 5
• Typical triad includes high-spiking fevers (>39°C, quotidian pattern), salmon-pink maculopapular rash (usually trunk/proximal limbs), and arthritis/arthralgias affecting knees, wrists, ankles, and elbows. 3
• The arm pain below the elbow fits the typical joint involvement pattern. 3
• Rash is usually evanescent and accompanied by fever, though vasculitic purpuric variants exist. 3

Hematologic Causes

Thrombocytopenia-Related

• Immune thrombocytopenic purpura can cause petechial rash. 1
• Obtain complete blood count with differential to assess platelet count, leukopenia, or bandemia. 1, 2
• Check comprehensive metabolic panel for hyponatremia and elevated hepatic transaminases. 1, 2

Infectious Causes (Non-Life-Threatening)

Viral Exanthems

• Enteroviruses, human herpesvirus 6, parvovirus B19, and Epstein-Barr virus can cause petechial rash. 1, 6
• Parvovirus B19 can cause petechiae in intertriginous areas (including inguinal region) beyond the classic "gloves and socks" distribution. 6
• Viral causes typically progress more slowly than bacterial infections. 1, 5

Mechanical/Iatrogenic Causes

Rumpel-Leede Phenomenon

• Petechial rash from acute dermal capillary rupture due to raised pressure in dermal vessels. 7
• Risk factors include diabetes mellitus, hypertension, thrombocytopenia, chronic steroid use, antiplatelets, and anticoagulants. 7
• Consider if recent blood pressure cuff inflation or mechanical trauma to the area. 7

Diagnostic Algorithm

Immediate Assessment (Within 1 Hour)

• Check vital signs for fever, tachycardia, hypotension, or altered mental status indicating systemic toxicity. 1
• Obtain detailed exposure history: tick exposure, recent trauma/surgery, IV drug use, indwelling catheters, travel to endemic areas, new medications (especially anticoagulants). 1, 2, 4
• Examine rash distribution: progression speed, blanching vs. non-blanching, palpable purpura, involvement of palms/soles (indicates advanced RMSF). 1

Laboratory Studies

• Complete blood count with differential and peripheral blood smear. 1, 2
• Comprehensive metabolic panel. 1, 2
• Coagulation studies (PT/INR, aPTT). 4
• Blood cultures before antibiotics if possible, but do not delay treatment. 1
• Acute serology for R. rickettsii, E. chaffeensis, A. phagocytophilum. 2

Treatment Decision Points

• If ANY concern for RMSF or meningococcemia: start doxycycline 100 mg twice daily immediately, add ceftriaxone if meningococcemia cannot be excluded. 1, 2
• Hospitalize if systemic toxicity, rapidly progressive rash, or diagnostic uncertainty between serious causes. 1
• Clinical improvement expected within 24-48 hours of doxycycline for rickettsial diseases. 2

Critical Pitfalls to Avoid

• Do not wait for the classic triad of fever, rash, and tick bite in RMSF—present in only a minority at initial presentation. 1
• Do not exclude serious disease based on absence of rash—up to 20% of RMSF cases and 50% of early meningococcal cases lack rash. 1
• Do not delay empiric antibiotics waiting for laboratory confirmation. 1
• Consider that absence of tick exposure does not exclude RMSF. 1