**Core Concept**
Nongonococcal septic arthritis refers to a joint infection caused by bacteria other than Neisseria gonorrhoeae. This condition is a medical emergency that requires prompt diagnosis and treatment to prevent joint damage and long-term disability. The most common causative organisms are typically gram-positive bacteria.

**Why the Correct Answer is Right**
Staphylococcus aureus is the most common cause of nongonococcal septic arthritis, accounting for approximately 60% of cases. This bacterium is a gram-positive cocci that can infect the joint through hematogenous spread from a distant site, such as the skin or lungs, or through direct inoculation during a traumatic injury. The bacteria can colonize the joint space and cause an intense inflammatory response, leading to joint destruction and disability.

**Why Each Wrong Option is Incorrect**
* **Option A:** Streptococcus pneumoniae is a gram-positive bacterium that can cause septic arthritis, but it is less common than Staphylococcus aureus. It is typically seen in patients with underlying conditions such as sickle cell disease or asplenia.
* **Option B:** Escherichia coli is a gram-negative bacterium that can cause septic arthritis, but it is more commonly associated with urinary tract infections. It is a less common cause of septic arthritis compared to Staphylococcus aureus.
* **Option D:** Klebsiella pneumoniae is a gram-negative bacterium that can cause septic arthritis, but it is rare and typically seen in patients with underlying conditions such as diabetes or immunocompromised states.

**Clinical Pearl / High-Yield Fact**
Staphylococcus aureus septic arthritis often presents with a sudden onset of joint pain and swelling, and a high fever. It is essential to suspect septic arthritis in patients with a history of recent trauma, injection drug use, or underlying medical conditions, and to obtain a joint aspirate for culture and Gram stain.

**Correct Answer:** C. Staphylococcus aureus.