Differential Diagnosis for Hard Skin Lesions

The patient presents with hard skin lesions resembling large urticaria, which are intermittent, painful upon touch and pressure, and leave black marks upon resolution. These lesions persist for more than a month without itching. Based on these symptoms, the following differential diagnoses are considered:

• Single Most Likely Diagnosis

  • Erythema Nodosum: This condition is characterized by the sudden onset of painful, erythematous nodules or plaques, typically on the lower extremities. The lesions can leave behind bruise-like marks as they resolve, which fits the description of black marks. The absence of itching and the painful nature of the lesions also align with erythema nodosum.

• Other Likely Diagnoses

  • Urticarial Vasculitis: This is a condition that presents with urticaria-like lesions but is actually a form of vasculitis affecting the small blood vessels of the skin. The lesions can be painful, last longer than 24 hours, and may leave behind pigmentation changes, which could be described as black marks.
  • Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): Characterized by the sudden onset of fever, leukocytosis, and tender, erythematous skin lesions. While itching is not a primary symptom, the lesions can be painful and may leave behind hyperpigmentation.

• Do Not Miss Diagnoses

  • Cutaneous Malignancy (e.g., Cutaneous T-Cell Lymphoma): Although less likely, certain cutaneous malignancies can present with persistent, painful skin lesions. The absence of systemic symptoms or significant lymphadenopathy does not rule out this possibility.
  • Infectious Diseases (e.g., Deep Fungal Infections, Mycobacterial Infections): Certain infectious diseases can cause chronic, painful skin lesions that may leave behind pigmentation changes. These conditions are critical to diagnose early due to their potential for serious morbidity.

• Rare Diagnoses

  • Erythema Elevatum Diutinum: A rare form of vasculitis that presents with persistent, erythematous to violaceous papules, plaques, or nodules, typically on the extensor surfaces. The lesions can be painful and may leave behind hyperpigmentation.
  • Necrobiosis Lipoidica: A rare condition characterized by necrotic, granulomatous lesions in the skin, often on the lower legs. These lesions can be painful and may leave behind atrophic, pigmented scars.

Each of these diagnoses requires careful consideration of the patient's full clinical presentation, including history, physical examination, and potentially, diagnostic tests such as skin biopsies, to determine the most appropriate diagnosis and treatment plan.