What are possible diagnoses for a 40-year-old female with progressive pain and swelling in her right leg, lymphadenopathy (lymph node enlargement) in the groin, erythema (redness) in the groin, and a purulent sore on her right buttock, with no recalled trauma or bite?

Differential Diagnosis for Progressive Leg Pain with Lymphadenopathy and Purulent Buttock Lesion

The most critical diagnosis to rule out immediately is Fournier's gangrene or necrotizing soft tissue infection (NSTI), given the purulent buttock lesion, groin involvement with lymphadenopathy, and progressive ascending symptoms—this requires emergency surgical consultation within hours. 1

Primary Diagnostic Considerations

Necrotizing Soft Tissue Infection / Fournier's Gangrene (MOST URGENT)

• The combination of a purulent sore on the buttock with groin erythema, lymphadenopathy, and ascending leg pain strongly suggests Fournier's gangrene, which originates from the perineal/perianal region in 24% of cases and can rapidly spread along fascial planes down into the legs 1
• Pain intensity often exceeds visible clinical findings early in the disease process, and cutaneous manifestations appear later as the infection progresses 1
• Mortality approaches 20-50% without early recognition and immediate surgical debridement 1
• The infection spreads through Colles' fascia (continuous with Scarpa's fascia) from the perineum down to the thighs, explaining the ascending pattern from foot to leg 1

Red flags requiring immediate surgical consultation: 2

• Rapid progression of infection (pain started in foot, now ascending)
• Pain out of proportion to findings
• Purulent discharge from buttock lesion
• Groin involvement with lymphadenopathy

Nodular Lymphangitis with Ascending Infection

• Nodular lymphangitis presents with inflammatory nodules along lymphatic channels draining a primary skin infection, typically following superficial inoculation 3, 4
• The purulent buttock sore could represent the primary inoculation site, with ascending lymphangitis causing the progressive leg symptoms 3
• Common causative organisms include Sporothrix schenckii (sporotrichosis), Nocardia brasiliensis, Mycobacterium marinum, Leishmania species, and Francisella tularensis 3, 4
• Recent travel history is critical—marine environment exposure suggests M. marinum, while gardening activities suggest sporotrichosis 3, 4
• Frankly purulent discharge from the primary lesion is associated with Francisella and Nocardia species 3, 4
• Incubation period of 1-5 days with painful chancre and prominent tender lymphadenitis strongly implicates tularemia 3

Severe Cellulitis with Lymphangitis

• Cellulitis requires at least 2 inflammatory signs: erythema, warmth, tenderness, pain, or induration 1, 2
• The groin erythema and leg pain/swelling meet criteria for moderate-to-severe cellulitis (erythema >2 cm with deeper tissue involvement and lymphadenopathy) 2
• Lymphangitic streaking extending from the buttock lesion toward groin lymph nodes would support this diagnosis 5
• However, the purulent buttock lesion as a portal of entry with ascending symptoms raises concern for more aggressive infection 1

Polymicrobial Perineal/Perianal Infection

• Infections originating from the perineal/perianal region are typically polymicrobial, involving aerobic and anaerobic organisms including Streptococcus, Staphylococcus, E. coli, and anaerobes 1, 6
• The buttock location near the perineum increases risk for gram-negative and polymicrobial pathogens 6
• These infections can progress to necrotizing fasciitis if not recognized early 1, 6

Deep Vein Thrombosis (Must Exclude)

• Calculate Wells score immediately to stratify DVT probability: recent travel, leg swelling, and pain warrant consideration 2
• If Wells score ≥2, proceed directly to complete duplex ultrasound 2
• However, the purulent buttock lesion, groin erythema, and lymphadenopathy strongly favor infectious etiology over isolated DVT 2

Immediate Diagnostic Workup

Laboratory studies: 1

• Complete blood count with differential (leukocytosis)
• C-reactive protein and erythrocyte sedimentation rate (elevated levels predict worse outcomes) 2, 7
• Basic metabolic panel (assess for metabolic instability)
• Blood cultures if systemic signs present
• Calculate LRINEC score (Laboratory Risk Indicator for Necrotizing Fasciitis) if NSTI suspected 1

Imaging: 1

• CT scan of pelvis and lower extremity to assess extent of soft tissue involvement, gas in tissues (crepitus), and perirectal/retroperitoneal extension
• Ultrasound to evaluate for abscess formation and rule out DVT 2

Microbiologic evaluation: 1

• Obtain cultures from purulent buttock lesion before antibiotics
• Deep tissue cultures if surgical debridement performed
• Consider fungal and mycobacterial cultures given travel history 3, 4

Critical Management Principles

Immediate actions: 1

• Emergency surgical consultation for evaluation of necrotizing infection—early and aggressive debridement improves survival
• Hemodynamic support and resuscitation if systemic instability present
• Broad-spectrum empirical antibiotics covering gram-positive, gram-negative, and anaerobic organisms

Empirical antibiotic regimen for suspected NSTI/Fournier's gangrene: 1, 6

• Vancomycin (for MRSA coverage) PLUS piperacillin-tazobactam or carbapenem (for gram-negatives and anaerobes)
• Tailor antibiotics based on culture results from surgical debridement

For cellulitis/lymphangitis without necrotizing features: 1, 2

• Empirical coverage for Staphylococcus and Streptococcus with consideration for MRSA risk factors
• If nodular lymphangitis suspected and patient fails empiric therapy, biopsy with appropriate stains and cultures for fungi, mycobacteria, and atypical organisms 3, 4

Common Pitfalls to Avoid

• Do not delay surgical consultation while awaiting imaging or laboratory results if clinical suspicion for necrotizing infection is high—this is a time-sensitive emergency 1
• Do not assume simple cellulitis when a purulent lesion in the buttock/perineal region is present with ascending symptoms—consider more aggressive infections 1
• Do not overlook travel history—specific geographic exposures guide consideration of endemic organisms causing nodular lymphangitis 3, 4
• Do not prescribe antibiotics alone for suspected necrotizing infection—surgical debridement is the definitive treatment 1