Differential Diagnosis for Painful Lumps on Arms and Legs

Single Most Likely Diagnosis

• Insect Bites or Stings: Given the acute onset and location of the painful lumps, insect bites or stings are a common and likely cause. The symptoms could be due to an allergic reaction or infection secondary to the bites.

Other Likely Diagnoses

• Folliculitis: An inflammation of the hair follicles, often caused by bacterial infection, which can present as painful lumps on the skin.
• Furuncles or Carbuncles: Boils or groups of boils that can develop on the skin, especially in areas prone to sweating or friction, and can be painful.
• Erythema Nodosum: A condition characterized by the sudden formation of painful nodules under the skin, often on the legs, which can be triggered by various factors including infections and medications.

Do Not Miss Diagnoses

• Cellulitis: A bacterial skin infection that can cause redness, swelling, and pain. It requires prompt antibiotic treatment to prevent serious complications.
• Necrotizing Fasciitis: A severe infection that destroys tissue under the skin, including fat and the tissue covering the muscles (fascia). It's a medical emergency.
• Deep Vein Thrombosis (DVT): Although less likely given the description of lumps, DVT can cause swelling and pain in the legs and is a potentially life-threatening condition if not treated promptly.

Rare Diagnoses

• Panniculitis: Inflammation of the subcutaneous fat, which can cause painful lumps under the skin.
• Weber-Christian Disease: A rare inflammatory disorder of the fatty tissues under the skin, characterized by recurring episodes of painful inflammation.
• Erythema Induratum: A form of panniculitis that can cause painful nodules, typically on the calves of the legs, often associated with tuberculosis or other systemic conditions.

Each of these diagnoses should be considered based on the patient's full clinical presentation, including history, physical examination, and potentially, diagnostic tests.