Differential Diagnosis for Orchitis, Fever, Weight Loss, and Rheumatoid Arthritis

Single Most Likely Diagnosis

• Reactive Arthritis (ReA): This condition is characterized by the development of arthritis following an infection, often of the urogenital or gastrointestinal tract. The presence of orchitis (inflammation of the testicles) and rheumatoid arthritis-like symptoms, along with systemic symptoms such as fever and weight loss, could be indicative of Reactive Arthritis, especially if triggered by a bacterial infection like Chlamydia or Salmonella.

Other Likely Diagnoses

• Sarcoidosis: This is a systemic granulomatous disease that can affect multiple organs, including the lungs, skin, eyes, and joints. It can cause arthritis similar to rheumatoid arthritis, fever, weight loss, and can also involve the testes, leading to orchitis. The multi-systemic nature of sarcoidosis makes it a plausible diagnosis.
• Lyme Disease: Caused by the bacterium Borrelia burgdorferi, Lyme disease can lead to arthritis, fever, and weight loss. While orchitis is less commonly associated with Lyme disease, the broad range of symptoms it can cause makes it a consideration in this differential diagnosis.

Do Not Miss Diagnoses

• Testicular Cancer with Paraneoplastic Syndrome: Although rare, testicular cancer can present with orchitis and systemic symptoms like fever and weight loss. A paraneoplastic syndrome could potentially cause rheumatoid arthritis-like symptoms. Missing this diagnosis could be fatal due to the aggressive nature of testicular cancer if left untreated.
• Tuberculosis (TB): TB can cause a wide range of symptoms, including fever, weight loss, and arthritis. Genitourinary TB can lead to orchitis. Given the potential severity and the fact that TB is a treatable condition, it is crucial not to miss this diagnosis.

Rare Diagnoses

• Whipple's Disease: A rare, systemic bacterial infection caused by Tropheryma whipplei, which can lead to arthritis, fever, weight loss, and less commonly, orchitis. The rarity of this condition makes it less likely but still a consideration in a differential diagnosis due to its systemic nature.
• Brucellosis: A zoonotic infection that can cause fever, weight loss, arthritis, and orchitis. While rare in many parts of the world, it should be considered, especially in individuals with exposure to infected animals or their products.