What is the most likely cause of the red‑brown macules on the palms and soles in a 40‑year‑old man with known knee psoriasis?

Secondary Syphilis

The red-brown macules on this patient's palms and soles that appeared a couple of weeks ago are most likely secondary syphilis, not psoriasis. This is a critical diagnostic pitfall that must be recognized immediately to prevent serious morbidity and mortality.

Key Clinical Reasoning

Why This is Secondary Syphilis, Not Psoriasis

The clinical presentation contains several red flags that distinguish this from palmoplantar psoriasis:

• Acute onset: The palmar and plantar lesions started "a couple weeks ago" - this is far too rapid for psoriasis, which typically develops gradually over months to years
• Red-brown macules: Secondary syphilis classically presents with copper-colored or red-brown macules on palms and soles
• Timing: The recent appearance (weeks) suggests an acute infectious process rather than chronic inflammatory disease
• Distribution pattern: While psoriasis can affect palms and soles, the acute bilateral symmetric involvement of these areas is a hallmark of secondary syphilis

Psoriasis Characteristics That Don't Fit

According to the guidelines 1, palmoplantar psoriasis typically presents with:

• Hyperkeratotic plaques with silvery scale and fissuring - not simple red-brown macules
• Chronic course developing over extended periods
• Significant keratoderma of palms and soles
• Erythematous scaly patches rather than smooth macules

The guidelines specifically note that "lack of significant keratoderma of his palms and soles" argues against psoriasis as the primary diagnosis 1.

Critical Differential Diagnosis

What Must Be Ruled Out Immediately

Secondary syphilis is the most urgent diagnosis to exclude because:

• It requires immediate antibiotic treatment to prevent tertiary complications (neurosyphilis, cardiovascular syphilis, gummatous disease)
• Untreated syphilis has significant mortality and morbidity
• The classic presentation includes palmoplantar macules appearing 4-10 weeks after primary infection
• It can mimic numerous dermatologic conditions, earning its nickname as "the great imitator"

Other Considerations

While the guidelines discuss PUVA-induced pigmented macules 2, 3, this patient has no history of phototherapy treatment, making this irrelevant.

Immediate Diagnostic Workup Required

1. Serologic testing: RPR or VDRL (nontreponemal) plus confirmatory treponemal testing (FTA-ABS or TP-PA)
2. Sexual history: Recent exposures, partners, symptoms of primary chancre
3. Complete skin examination: Look for other signs of secondary syphilis (condyloma lata, mucous patches, generalized rash, lymphadenopathy)
4. HIV testing: Given the association between syphilis and HIV

Treatment Implications

If syphilis is confirmed:

• Benzathine penicillin G 2.4 million units IM (single dose for early syphilis)
• This is curative and prevents progression to tertiary disease
• The knee psoriasis can then be addressed separately with appropriate topical or systemic therapy as outlined in the guidelines 1

Common Pitfall to Avoid

Do not assume all skin findings in a patient with known psoriasis are psoriatic lesions. The acute onset of palmoplantar macules in a 40-year-old male warrants immediate consideration of secondary syphilis, regardless of pre-existing psoriasis. Missing this diagnosis has serious consequences for both the patient and their sexual contacts.

The orthopedist correctly identified psoriasis at the knees, but the palmar and solar lesions represent a separate, more urgent pathology requiring immediate serologic evaluation and treatment.