Differential Diagnosis for Left Knee Tenderness
Given the history of reactive arthritis from dirty needle use and recent right arm cellulitis, the following differential diagnoses are considered:
- Single most likely diagnosis
- Reactive arthritis: This is the most likely diagnosis given the patient's history of reactive arthritis triggered by dirty needle use. The recent episode of right arm cellulitis could be a new trigger or a complication, and the left knee tenderness could be a manifestation of the reactive arthritis.
- Other Likely diagnoses
- Septic arthritis: The recent history of cellulitis and the use of dirty needles increase the risk of septic arthritis, especially if the left knee is involved.
- Osteomyelitis: Although less common, osteomyelitis could be a consideration, especially if the patient has a history of intravenous drug use or recent infection.
- Crystal arthropathy (gout or pseudogout): These conditions can cause acute knee pain and should be considered, especially if there are risk factors such as kidney disease or recent surgery.
- Do Not Miss diagnoses
- Septic knee joint from hematogenous spread: This is a medical emergency that requires prompt diagnosis and treatment to prevent serious complications.
- Necrotizing fasciitis: Although rare, this condition can occur in the setting of recent cellulitis and intravenous drug use, and it is a medical emergency.
- Endocarditis: Given the history of intravenous drug use, endocarditis should be considered, especially if there are systemic symptoms such as fever, chills, or heart murmur.
- Rare diagnoses
- Rheumatoid arthritis: Although less likely, rheumatoid arthritis could be a consideration, especially if there are other joint involvements or systemic symptoms.
- Lyme disease: If the patient has a history of tick exposure, Lyme disease could be a rare cause of knee pain and should be considered.
- Sarcoidosis: This condition can cause joint pain and swelling, but it is less common and would require further evaluation to diagnose.