Most common cause of nongonococcal septic arthritis is
**Core Concept**
Nongonococcal septic arthritis is a severe infection of the joint space caused by bacteria other than Neisseria gonorrhoeae. The most common causative organisms are Staphylococcus aureus and Streptococcus pneumoniae, which can lead to significant morbidity and mortality if not promptly treated.
**Why the Correct Answer is Right**
Staphylococcus aureus is the most common cause of nongonococcal septic arthritis, particularly in adults. This bacterium can infect the joint through hematogenous spread from a distant site, such as the skin, respiratory tract, or urinary tract. The mechanism involves the colonization of the joint space by S. aureus, leading to the production of virulence factors, including toxins and enzymes, which damage the cartilage and synovium, resulting in joint destruction and inflammation.
**Why Each Wrong Option is Incorrect**
* **Option A:** Although Streptococcus pneumoniae is a common cause of bacterial meningitis and pneumonia, it is less commonly associated with septic arthritis compared to S. aureus.
* **Option B:** Escherichia coli is a common cause of urinary tract infections but is less commonly associated with septic arthritis.
* **Option D:** Klebsiella pneumoniae is a cause of various infections, including pneumonia and urinary tract infections, but is not a common cause of septic arthritis.
**Clinical Pearl / High-Yield Fact**
The most common age group affected by nongonococcal septic arthritis is adults between the ages of 20 and 50, with a slight male predominance.
**Correct Answer: C. Staphylococcus aureus**