Differential Diagnosis

The patient's presentation is complex, with multiple potential causes for her symptoms. Here's a categorized differential diagnosis:

• Single most likely diagnosis
  • Septic arthritis of the left knee: The patient's recent history of hydrocortisone injections to her left knee, worsening knee pain, erythema, and swelling, along with systemic signs of infection (fever, leukocytosis, and reactive thrombocytosis), make septic arthritis a highly likely diagnosis. The patient's background of severe osteoarthritis (OA) and recent steroid injection increase her risk for this condition.
• Other Likely diagnoses
  • Atrial fibrillation with rapid ventricular response (RVR) exacerbation: The patient's known history of atrial fibrillation and current presentation of RVR could be contributing to her symptoms, potentially exacerbated by the stress of an infection or other systemic condition.
  • Pneumonia or atelectasis: Although the chest X-ray shows improvement, the presence of medial basilar consolidation could still indicate an infectious process, especially given the patient's fever and leukocytosis.
  • Exacerbation of severe bronchial asthma: While the patient denies shortness of breath, her history of severe asthma and current systemic symptoms could suggest an asthma exacerbation, though this seems less likely given the absence of respiratory symptoms.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Endocarditis: Given the patient's history of atrial fibrillation and current presentation with fever and systemic symptoms, endocarditis is a critical diagnosis not to miss, despite its lower likelihood.
  • Sepsis from another source: The patient's fever, leukocytosis, and thrombocytosis could indicate sepsis from a source other than the knee, such as a urinary tract infection, pneumonia, or intra-abdominal infection, which would require prompt identification and treatment.
  • Pulmonary embolism: Although the patient denies chest pain and shortness of breath, pulmonary embolism is a condition that can present subtly, especially in elderly patients, and is critical not to miss due to its high mortality rate.
• Rare diagnoses
  • Crystal arthropathy (gout or pseudogout): While less likely given the patient's recent knee injection and systemic symptoms suggestive of infection, crystal arthropathy could cause severe knee pain and should be considered, especially if the diagnosis of septic arthritis is not confirmed.
  • Rheumatologic flare: A flare of an underlying rheumatologic condition (if present) could cause systemic symptoms and knee pain, though this seems less likely given the patient's known history and current presentation.