What is the likely diagnosis and next steps for a 24-year-old female patient with a history of leptospirosis, frequent urinary tract infections (UTIs), and hypokalemia, presenting with foot rashes, multiple petechial-like lesions, and a large brown patch on the leg?

Likely Diagnosis: Post-Leptospirosis Pigmented Purpuric Dermatosis with Possible Underlying Vasculitis

The most likely diagnosis is a pigmented purpuric dermatosis (capillaritis) that developed following her leptospirosis infection, with the petechial-like lesions representing ongoing capillary inflammation and the brown patch representing hemosiderin deposition from chronic extravasation of red blood cells. The constellation of petechiae around the distal legs, hypokalemia, and history of leptospirosis strongly suggests persistent microvascular injury.

Critical Diagnostic Considerations

Leptospirosis-Related Sequelae

• Leptospirosis characteristically causes hypokalemia due to renal potassium wasting, which is potentiated by aldosterone and cortisol, requiring special attention to potassium replacement 1
• The persistent hypokalemia in this patient suggests ongoing tubular dysfunction, a hallmark of leptospirosis renal disease that can persist long after acute infection 2, 3
• Leptospirosis causes capillary fragility and hemorrhagic manifestations, which could explain the petechial lesions 4
• Asymptomatic leptospira renal colonization is an overlooked risk for chronic kidney disease and may progress insidiously with tubulointerstitial nephritis 5

Vasculitic Differential

• Petechiae around joints require immediate assessment for systemic toxicity and fever to exclude rapidly progressive infections 6
• The distribution of petechiae on distal legs with coalescence into a brown patch suggests a chronic pigmented purpuric dermatosis rather than acute vasculitis 6
• The absence of systemic symptoms (no fever mentioned currently) makes life-threatening infections like meningococcemia or necrotizing fasciitis less likely 6

Recurrent UTI Pattern

• Frequent UTIs combined with hypokalemia and history of leptospirosis raises concern for chronic tubulointerstitial disease with proximal tubule dysfunction 2, 5
• This pattern is characteristic of leptospirosis renal disease, which shows tubulointerstitial nephritis, interstitial fibrosis, and tubular atrophy 5

Immediate Next Steps

1. Rule Out Active Infection

• Obtain blood cultures if any fever or systemic toxicity is present 6
• Complete blood count with platelet count to exclude thrombocytopenia 6
• Assess for systemic inflammatory response syndrome criteria (temperature >38°C or <36°C, heart rate >90, respiratory rate >20, WBC >12,000 or <4,000) 7

2. Assess Renal Function and Electrolytes

• Comprehensive metabolic panel focusing on serum creatinine, potassium, and sodium levels 2, 3
• Urinalysis with microscopy looking for proteinuria, hematuria, and casts 4
• Calculate fractional excretion of sodium and potassium to assess tubular function 2, 1
• Serum aldosterone and cortisol levels if hypokalemia persists despite replacement 1

3. Dermatologic Evaluation

• Skin biopsy of both the petechial lesions and the brown patch for histopathology and direct immunofluorescence 7
• This will differentiate between pigmented purpuric dermatosis (showing perivascular lymphocytic infiltrate with hemosiderin deposition) versus true vasculitis (showing leukocytoclastic vasculitis with fibrinoid necrosis)
• Biopsy should be performed early in management when immune-mediated disease is suspected 7

4. Leptospirosis Follow-Up Testing

• Leptospira serology (IgM ELISA and MAT) to assess for persistent or recurrent infection 4
• Note that serology may remain positive for months after acute infection, but rising titers suggest reinfection 8, 4
• Consider PCR testing if available, though sensitivity decreases after the acute phase 8

5. Coagulation Assessment

• Coagulation profile (PT, PTT, fibrinogen) if bleeding disorder is suspected 6
• This is particularly important given the hemorrhagic manifestations associated with severe leptospirosis 4

Treatment Algorithm

If Active Leptospirosis Suspected

• Start antibiotics immediately upon clinical suspicion without waiting for laboratory confirmation 8, 4
• For mild disease: Doxycycline 100mg PO BID or Penicillin 8, 4
• For severe disease: IV Penicillin G or Ceftriaxone with intensive supportive care 4

If Pigmented Purpuric Dermatosis Confirmed

• Topical corticosteroids for symptomatic relief
• Compression therapy if venous insufficiency is contributing
• Avoid trauma to affected areas
• Monitor for progression

Hypokalemia Management

• Aggressive potassium replacement with close monitoring, as leptospirosis-induced hypokalemia requires special attention 1
• Address underlying tubular dysfunction
• Consider potassium-sparing diuretics if aldosterone-mediated wasting is confirmed 1

Recurrent UTI Management

• Urine culture to guide antibiotic therapy
• Evaluate for structural abnormalities if not previously done
• Consider prophylactic antibiotics if recurrences continue

Critical Pitfalls to Avoid

• Do not delay treatment while awaiting laboratory confirmation if active leptospirosis is suspected, as this can lead to life-threatening complications 8, 4
• Do not assume the brown patch is simply post-inflammatory hyperpigmentation without biopsy confirmation 7
• Do not ignore persistent hypokalemia, as it indicates ongoing tubular dysfunction that may progress to chronic kidney disease 2, 5
• Do not miss the opportunity to diagnose chronic leptospirosis renal colonization, which can insidiously progress to CKD 5
• If petechiae are widespread with systemic toxicity, immediately consider necrotizing fasciitis or other life-threatening infections 6

Long-Term Monitoring

• Serial renal function monitoring (creatinine, eGFR) every 3-6 months, as leptospirosis may predispose to chronic kidney disease 5
• Annual urinalysis to detect progressive proteinuria or hematuria 5
• Electrolyte monitoring, particularly potassium, until tubular function normalizes 2, 1
• Dermatologic follow-up to monitor for progression or new lesions