Differential Diagnosis for a 49-year-old Patient with Intermittent Fever and Lower Limb Swelling

Single Most Likely Diagnosis

• Cellulitis or Skin Infection: Given the symptoms of fever on and off for 15 days and lower limb swelling for 4 days, a bacterial skin infection such as cellulitis is a plausible diagnosis. The intermittent fever could be indicative of an ongoing infection, and the swelling in the lower limb could be due to the spread of infection or inflammation.

Other Likely Diagnoses

• Deep Vein Thrombosis (DVT): The swelling in the lower limb could also suggest DVT, especially if the patient has risk factors such as recent immobility, surgery, or cancer. Fever can sometimes accompany DVT, although it's not a universal symptom.
• Lymphangitis: This is an infection of the lymphatic vessels, which could cause fever and swelling in the affected limb. It often presents with red streaks from the infected area towards the lymph nodes.
• Osteomyelitis: An infection of the bone, which could cause intermittent fever and swelling over the affected bone, especially if it's in the lower limb.

Do Not Miss Diagnoses

• Septic Arthritis: Although less common, septic arthritis is a serious condition that requires prompt diagnosis and treatment. It could present with fever, swelling, and pain in a joint, often in the lower limbs.
• Necrotizing Fasciitis: A severe infection of the fascia, which could present with fever, swelling, and severe pain in the affected limb. It's a medical emergency due to its rapid progression and potential for severe complications.
• Malignancy (e.g., Lymphoma): In some cases, persistent fever and swelling could be indicative of an underlying malignancy, such as lymphoma. Although less likely, it's crucial not to miss this diagnosis due to its significant implications for treatment and prognosis.

Rare Diagnoses

• Erythema Nodosum: A condition characterized by the sudden formation of painful nodules under the skin, often on the lower limbs. It can be associated with fever and is sometimes linked to underlying infections or autoimmune conditions.
• Weber-Christian Disease: A rare inflammatory disease of the fatty tissues under the skin, which can cause recurring episodes of fever and inflammation in the subcutaneous fat, leading to swelling and pain.

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