Diagnosis: Secondary Syphilis
The most likely diagnosis is secondary syphilis, not a progression of psoriasis. The acute onset of red and brown macules on the palms and soles over just two weeks in a patient with pre-existing knee psoriasis strongly suggests a new infectious process rather than extension of the chronic psoriatic disease 1.
Key Diagnostic Reasoning
Why This is Secondary Syphilis
The acute 2-week timeline is incompatible with psoriasis, which follows a chronic, stable course or gradual progression over months to years, not days to weeks 1
The morphology is wrong for palmoplantar psoriasis: Psoriatic involvement of palms and soles presents as hyperkeratotic, fissured, scaly plaques with silvery scale—not red-brown macules 1, 2, 3
Classic syphilitic presentation: The symmetric distribution of copper-colored (red-brown) macules on palms and soles is pathognomonic for secondary syphilis 1
Coexistence of two conditions: The patient's pre-existing knee psoriasis is a separate, chronic condition that does not preclude acquiring syphilis 1
Why This is NOT Psoriasis Extension
Palmoplantar psoriasis would show thick, hyperkeratotic plaques with significant scaling and fissuring, not flat macules 3, 4
The acute onset over 2 weeks contradicts the natural history of psoriasis, which typically evolves gradually 1
Atopic or seborrheic dermatitis do not characteristically involve palms and soles with this morphology 1
Immediate Diagnostic Workup Required
Order RPR/VDRL and treponemal-specific testing (FTA-ABS or TP-PA) to confirm syphilis diagnosis 1
Screen for HIV as syphilis and HIV frequently coexist 5
Perform skin biopsy if diagnosis remains uncertain, though serologic testing is more definitive for syphilis 1
Take complete sexual history including number of partners, condom use, and symptoms in partners 1
Critical Management Steps
Partner notification and treatment is mandatory once syphilis is confirmed 1
Treatment: Benzathine penicillin G 2.4 million units IM as a single dose is the standard treatment for secondary syphilis 1
Continue managing the knee psoriasis separately with appropriate topical therapies (high-potency corticosteroids ± vitamin D analogues) 3
Common Pitfall to Avoid
The major diagnostic error would be attributing these new palmoplantar lesions to progression of the patient's known psoriasis and initiating systemic immunosuppressive therapy (methotrexate, biologics, cyclosporine) without first ruling out secondary syphilis 1, 3. This would be catastrophic, as immunosuppression could allow progression to tertiary syphilis with devastating neurologic and cardiovascular complications.