Differentials

1. Cutaneous small-vessel vasculitis / leukocytoclastic vasculitis
   • Often affects both lower legs or ankles.
   • Classically causes non-blanching, sometimes raised palpable purpura. It may be non-itchy.
   • Can follow an infection or be triggered by medications, autoimmune disease, or occur without a clear cause.
   • Important associated features: joint pain, abdominal pain, blood in urine, reduced urine, cough or breathlessness. Lower-extremity non-itchy non-blanching lesions are a pattern described with small-vessel vasculitis (Rosen's Emergency Medicine).
2. IgA vasculitis
   • A form of small-vessel vasculitis, particularly if there is purpura on the legs with abdominal pain, joint pains, or kidney involvement.
   • Can occur in adults, where renal assessment is important.
3. Petechiae/purpura from a platelet or clotting disorder
   • Low platelets, medication effects, viral illness, or other bleeding disorders can produce non-blanching spots, often on dependent areas such as the legs.
   • Needs a platelet count and coagulation testing if lesions are petechial/purpuric.
4. Infection-associated rash, including early meningococcal disease
   • Less typical if there is truly no fever and the person feels otherwise well, but headache plus a non-blanching rash requires prompt exclusion.
   • Early meningococcal lesions can begin as macules or maculopapules before becoming petechial/purpuric. The CDC clinical guidance notes that meningococcal bloodstream infection may present atypically.
5. Contact dermatitis or arthropod-bite reaction
   • This is possible if there was new footwear, socks, topical products, plants, workplace exposure, or insect exposure.
   • However, these are usually itchy. A blister can occur with allergic contact dermatitis or insect bites, but the lack of itch and the systemic symptoms make this less reassuring.
6. Friction blister or local trauma
   • The isolated left-foot blister may be unrelated to the ankle rash, especially with tight footwear, prolonged walking, heat, or rubbing.
   • Do not intentionally pop it. Keep it clean, dry, and covered.
7. Bullous dermatosis or bullous drug eruption
   • Consider if blisters multiply, skin becomes painful, there are mouth/eye/genital sores, skin peeling, or a new medication was started in the past days to weeks.
   • These need urgent review, especially with mucosal involvement.

What clinicians should check

• Whether lesions are blanching or non-blanching, flat or palpable, and whether they are spreading.
• Vital signs and neurologic examination, including neck stiffness.
• CBC with platelet count, renal function, urinalysis, inflammatory markers, liver tests, and coagulation profile.
• Depending on findings: blood cultures, viral/infection testing, autoimmune/vasculitis work-up, and possibly skin biopsy of a fresh lesion.