A 33-year-old sexually active woman, a week after her menstrual period, develops a painful, swollen right knee. The most likely causative organism has which of the following characteristics?
**Core Concept**
The patient's symptoms are suggestive of a bacterial infection, specifically septic arthritis, which can occur in the knee joint. The causative organism is likely a bacterium that has entered the joint space through a breach in the synovial membrane.
**Why the Correct Answer is Right**
The most common causative organism of septic arthritis in adults is *Staphylococcus aureus*. This bacterium has a unique characteristic: it can produce a coagulase enzyme, which helps it to evade the host's immune system. Additionally, *S. aureus* can form biofilms, which make it resistant to antibiotics and host immune cells. The patient's symptoms of pain and swelling in the knee joint, combined with the presence of a breach in the synovial membrane (e.g., a recent injury or surgical procedure), make *S. aureus* the most likely causative organism.
**Why Each Wrong Option is Incorrect**
**Option A:** *Neisseria gonorrhoeae* is a causative organism of septic arthritis, but it is more common in sexually active young women, particularly during their menstrual period. However, it is less common than *S. aureus* in this patient population.
**Option B:** *Escherichia coli* is a common cause of urinary tract infections, but it is not typically associated with septic arthritis.
**Option C:** *Klebsiella pneumoniae* is a gram-negative rod that can cause a variety of infections, including pneumonia and urinary tract infections, but it is not typically associated with septic arthritis.
**Clinical Pearl / High-Yield Fact**
*Staphylococcus aureus* is a common cause of septic arthritis in adults, particularly in the setting of a breach in the synovial membrane. This organism can produce a coagulase enzyme, which helps it to evade the host's immune system, and can form biofilms, making it resistant to antibiotics.
**Correct Answer:** C. *Staphylococcus aureus*