Differential Diagnosis

The patient presents with on and off pain in the right thigh, mild swelling in the right inguinal region, and a history of obesity. Given the negative filariasis tests, we can consider other potential causes for his symptoms.

• Single Most Likely Diagnosis
  • Lymphedema or Chronic Venous Insufficiency: Given the patient's obesity and the location of the swelling in the inguinal region, lymphedema or chronic venous insufficiency could be a plausible explanation. The swelling has been present for 6 months, which suggests a chronic condition.
• Other Likely Diagnoses
  • Cellulitis or Skin Infection: The patient has been taking doxycycline for the last 7 days, which might indicate a suspected bacterial infection. Cellulitis could cause pain and swelling in the affected area.
  • Deep Vein Thrombosis (DVT): Although less likely given the chronic nature of the symptoms, DVT should be considered, especially in an obese patient. However, the absence of more severe symptoms like significant swelling, warmth, or discoloration makes this less likely.
  • Lipoma or Soft Tissue Tumor: A benign growth like a lipoma could cause localized swelling and pain, especially if it is large enough to compress surrounding tissues.
• Do Not Miss Diagnoses
  • Testicular Cancer with Metastasis: Although rare, testicular cancer can metastasize to the inguinal lymph nodes, causing swelling. Given the patient's age and symptoms, this is a critical diagnosis not to miss.
  • Lymphoma: Lymphoma can cause lymphadenopathy (swelling of the lymph nodes) in the inguinal region, among other areas. This would be a serious diagnosis with significant implications for treatment and prognosis.
• Rare Diagnoses
  • Panniculitis: An inflammation of the subcutaneous fat, which could cause pain and swelling in the affected area. This condition is less common and might be considered if other diagnoses are ruled out.
  • Filarial Disease with False-Negative Tests: Although the patient's filariasis tests are negative, false negatives can occur. If there's a strong clinical suspicion based on exposure history and symptoms, further investigation might be warranted.

Each of these diagnoses should be considered in the context of the patient's full clinical picture, including further history, physical examination, and potentially additional diagnostic tests to determine the underlying cause of his symptoms.