Differential Diagnosis for Raised Indurated Skin Lesions on Toes

Single Most Likely Diagnosis

• Chronic discoid lupus erythematosus or other autoimmune conditions: Given the patient's history of Ross River virus, which can trigger autoimmune responses, and the description of raised indurated skin lesions, an autoimmune condition such as chronic discoid lupus erythematosus could be a likely diagnosis. However, without more specific information, this remains speculative.

Other Likely Diagnoses

• Squamous cell carcinoma or other skin cancers: In an 89-year-old patient, skin cancers are a common consideration for new or changing skin lesions, especially on sun-exposed areas like the toes, although the toes are less common than other areas.
• Eczema (nummular or stasis dermatitis): Eczematous conditions can present with raised, indurated lesions, especially in the context of venous insufficiency or localized irritation.
• Insect bite reactions or contact dermatitis: Reactions to insect bites or contact with irritants can cause raised, indurated lesions.

Do Not Miss Diagnoses

• Malignancy (e.g., melanoma, basal cell carcinoma): Although less likely given the description, any new or changing skin lesion in an elderly patient warrants consideration of malignancy due to the potential for serious consequences if missed.
• Infectious causes (e.g., mycobacterial infections, deep fungal infections): Infections such as mycobacterial or deep fungal infections can present with indurated skin lesions and have significant implications if not treated properly.
• Vasculitis: Certain forms of vasculitis can present with skin lesions and have a wide range of systemic implications.

Rare Diagnoses

• Panniculitis: An inflammation of the subcutaneous fat, which can present with indurated skin lesions.
• Erythema induratum (Bazin disease): A rare condition associated with tuberculosis, characterized by recurring nodules or plaques on the calves, which could potentially appear on the toes.
• Necrobiosis lipoidica: A rare condition characterized by necrotic, granulomatous lesions in the skin, often on the lower legs.

Each of these diagnoses would require further investigation, including a thorough history, physical examination, and potentially diagnostic tests such as biopsies, to determine the underlying cause of the patient's symptoms.