Differential Diagnosis for Sudden Onset Knee Swelling
- Single Most Likely Diagnosis
- Gonococcal Arthritis: This is a likely diagnosis given the patient's age, sexual activity, and the acute onset of symptoms without a history of trauma. The use of oral contraceptives may also increase the risk of gonococcal infection. The presentation of sudden onset knee swelling, fever, and the absence of trauma points towards a septic arthritis, with Neisseria gonorrhoeae being a common cause in sexually active young adults.
- Other Likely Diagnoses
- Staphylococcal Arthritis: Staphylococcus aureus is a common cause of septic arthritis and can occur without a clear source of infection. The patient's symptoms of fever, knee swelling, and decreased range of motion are consistent with this diagnosis.
- Lyme Arthritis: Although less common, Lyme disease can cause arthritis, particularly in endemic areas. However, the acute onset and systemic symptoms such as fever might be less typical for Lyme arthritis, which often presents with more chronic symptoms.
- Do Not Miss Diagnoses
- Meningococcal Arthritis: While rare, Neisseria meningitidis can cause septic arthritis, and given its potential for severe consequences, including rapid progression to sepsis and death, it is crucial not to miss this diagnosis. The patient's fever and acute onset of symptoms could be consistent with this, although it is less likely than gonococcal arthritis.
- Crystal Arthropathy (Gout or Pseudogout): Although these conditions are more common in older adults, they can occur in younger individuals. The sudden onset of severe joint pain and swelling could be indicative of a crystal arthropathy, and missing this diagnosis could lead to inappropriate treatment.
- Rare Diagnoses
- Tuberculous Arthritis: This is a rare cause of septic arthritis in developed countries but should be considered in patients with risk factors for tuberculosis or those who have recently traveled to endemic areas. The presentation can be similar to other forms of septic arthritis, with chronic symptoms being more common.
- Fungal Arthritis: Fungal infections are rare causes of arthritis and typically occur in immunocompromised individuals. However, given the potential severity of fungal infections, they should be considered in the differential diagnosis, especially if the patient has risk factors such as immunosuppression.