Differential Diagnosis for the 30-year-old Male with Knee Pain and Recent Diarrhea

• Single most likely diagnosis:
  • Reactive arthritis: This condition is characterized by arthritis that occurs after an infection elsewhere in the body, often in the gastrointestinal tract or the urethra. The patient's recent history of diarrhea, which was caused by C. difficile, followed by knee pain, swelling, erythema, and systemic symptoms like fever, suggests a reactive arthritis. The elevated WBC, ESR, and CRP support an inflammatory process.
• Other Likely diagnoses:
  • Septic arthritis: Although less likely given the context of a recent gastrointestinal infection, septic arthritis is a consideration, especially with the presence of fever, elevated WBC, and localized pain. However, the absence of a more direct infectious source (e.g., recent trauma, injection drug use) and the specific pattern of symptoms (e.g., pain at the Achilles tendon insertion) make it less likely than reactive arthritis.
  • Peripheral joint related to IBD: Inflammatory bowel disease (IBD) can cause peripheral arthritis, but the patient's symptoms and recent history of C. difficile infection do not strongly suggest chronic IBD as the primary diagnosis at this point.
• Do Not Miss diagnoses:
  • Gonococcus: Gonococcal arthritis is a critical diagnosis not to miss due to its potential for severe consequences if untreated, including destruction of the joint. However, the patient's presentation and recent history of diarrhea due to C. difficile make this less likely. It's essential to consider gonococcal arthritis in any young adult with acute monoarthritis, especially if there's a risk of sexually transmitted infections.
• Rare diagnoses:
  • Other rare causes of arthritis, such as rheumatoid arthritis or lupus, could be considered but are less likely given the acute onset and the specific recent history of gastrointestinal infection. These conditions typically have a more chronic course and different patterns of joint involvement.