Differential Diagnosis

The patient presents with worsening pain in the left hip/groin, fever, and elevated inflammatory markers. Based on the provided information, the differential diagnosis can be categorized as follows:

• Single Most Likely Diagnosis
  • Septic arthritis of the left hip: The patient's symptoms of worsening hip pain, fever (39.5°C), elevated CRP (193), and WBC count (12.2) are highly suggestive of a septic joint. The recent history of hip replacement increases the risk of prosthetic joint infection. The CT scan findings of slight thickening in the left iliacus muscle with an increasing infective process support this diagnosis.
• Other Likely Diagnoses
  • Prosthetic joint infection: Given the patient's history of left hip replacement, prosthetic joint infection is a strong consideration, especially with the presence of fever, elevated inflammatory markers, and localized pain.
  • Psoas abscess: The CT scan findings of thickening in the left iliacus muscle could also suggest a psoas abscess, which is an infection involving the psoas muscle. This condition can cause hip pain and fever.
  • Gout: Although the patient has a history of gout, the acute presentation with fever and significantly elevated inflammatory markers makes this less likely as the primary diagnosis at this time.
• Do Not Miss Diagnoses
  • Osteomyelitis: Infection of the bone, which could be related to the hip replacement or a separate process. It's crucial to consider this diagnosis due to its potential for serious complications if not promptly treated.
  • Retroperitoneal hemorrhage: Although the patient is on anticoagulation (Rivaroxaban), the absence of trauma and the presence of infection signs (fever, elevated CRP) make this less likely. However, it's a critical diagnosis not to miss due to its high mortality rate if not promptly addressed.
• Rare Diagnoses
  • Tuberculous arthritis or osteomyelitis: Although rare in this demographic, tuberculosis can cause joint or bone infections, which might present with chronic pain, fever, and elevated inflammatory markers.
  • Fungal infections: In immunocompromised patients or those with specific risk factors, fungal infections can cause similar symptoms. However, there's no clear indication of immunocompromise in this patient beyond his age and medical conditions.

Next Best Step in Management

Given the most likely diagnosis of septic arthritis of the left hip, the next best steps would include:

• Immediate antibiotic therapy: Broad-spectrum antibiotics should be started as soon as possible, with consideration for coverage of common pathogens causing prosthetic joint infections, including Staphylococcus aureus and coagulase-negative staphylococci.
• Orthopedic consultation: An urgent consultation with an orthopedic surgeon is necessary to discuss the potential need for surgical intervention, such as drainage or removal of the prosthetic joint.
• Further diagnostic workup: This may include blood cultures, joint aspiration for culture and analysis, and potentially additional imaging to fully assess the extent of the infection.
• Management of anticoagulation: The patient's anticoagulation therapy may need to be adjusted in the context of potential surgical intervention or to manage the risk of bleeding complications.