What is the likely diagnosis and recommended work‑up for a solitary 5‑mm maculopustular papule on the dorsal proximal phalanx with no other skin involvement?

Likely Diagnosis: Solitary Cutaneous Mastocytoma

For a solitary 5-mm maculopustular papule on the dorsal proximal phalanx with no other skin involvement, the most likely diagnosis is a solitary cutaneous mastocytoma, and the essential diagnostic maneuver is to stroke or rub the lesion to elicit Darier's sign (urticaria/wheal formation), which confirms the diagnosis clinically without requiring biopsy in typical presentations 1, 2.

Clinical Approach

Initial Bedside Assessment

Perform the Darier sign test immediately:

• Firmly stroke or rub the lesion
• Positive test: urticaria, wheal, or blister formation within minutes
• This physical finding is pathognomonic for mastocytosis 1

Examine for:

• Lesion characteristics: indurated, erythematous, yellow-brown or reddish-brown appearance
• Peau d'orange (orange peel) texture
• Leathery or rubbery consistency
• Size typically ≤5 cm 1

Critical Exclusions

Rule out systemic involvement (which would be absent in solitary mastocytoma):

• Palpate for hepatosplenomegaly (should be absent)
• Check for lymphadenopathy (should be absent)
• Assess for additional skin lesions on full body examination 1

Differential Diagnosis Considerations

While mastocytoma is most likely given the location and solitary nature, briefly consider:

Malignant possibilities requiring biopsy if features present:

• Amelanotic melanoma: Look for asymmetry, irregular borders, color heterogeneity, diameter >6mm, evolution/recent change 3
• Merkel cell carcinoma: Rapidly growing pink papule, though rare on digits 4

Benign alternatives:

• Hemangioma: Dermoscopy would show benign vascular patterns 4
• Epidermolytic acanthoma: Hyperkeratotic papule, more common on trunk 5

Diagnostic Work-Up Algorithm

If Darier Sign is POSITIVE:

1. No biopsy required - diagnosis is clinical 1
2. No laboratory studies needed for solitary lesions 1
3. Provide parental reassurance regarding spontaneous resolution (typically before puberty if onset in first 2 years of life) 1

If Darier Sign is NEGATIVE or Uncertain:

1. Excisional biopsy is mandatory to rule out melanoma 3, 6
   • Use narrow (1-3 mm) margin excision
   • Full-thickness removal extending into deep reticular dermis
   • Avoid shave biopsy that might transect the base
   • Orient specimen with long axis parallel to skin lines for potential re-excision 3, 6

If ANY Melanoma Features Present:

Proceed directly to excisional biopsy with 2mm margins 3:

• Asymmetry
• Irregular borders
• Heterogeneous color
• Recent change in size, shape, or color
• Bleeding or ulceration

Management

For Confirmed Mastocytoma:

• Symptomatic treatment only 1
• Avoid triggering factors (friction, temperature extremes, certain medications)
• Antihistamines for pruritus if needed
• Reassurance: spontaneous resolution expected before puberty in lesions appearing within first 2 years of life 1

Common Pitfalls to Avoid:

1. Do not dismiss a solitary pink/red papule on a digit without proper evaluation - amelanotic melanoma and Merkel cell carcinoma can present this way 4

2. Do not perform superficial shave biopsy if melanoma is in differential - this underestimates Breslow thickness and compromises staging 6, 7

3. Do not skip the Darier sign test - this simple bedside maneuver can prevent unnecessary biopsies 1

4. Do not order systemic work-up for solitary mastocytoma - organomegaly and systemic involvement are characteristically absent, and laboratory studies are not indicated 1

The unique case report of a divided mastocytoma on the dorsal hand extending to proximal phalanges 2 supports that this location, while unusual, is consistent with solitary mastocytoma when other features align.