**Core Concept**
Septic arthritis is a medical emergency characterized by inflammation of a joint due to an infectious agent. It typically involves the invasion of the joint space by bacteria, fungi, or other microorganisms, leading to purulent joint fluid and potentially severe joint damage.

**Why the Correct Answer is Right**
The most common cause of septic arthritis in adults is *Staphylococcus aureus*, a gram-positive bacterium that can enter the joint through a variety of routes, including hematogenous spread from a distant infection, direct inoculation from a skin wound or surgical procedure, or contiguous spread from an adjacent infected structure. The bacteria can colonize the synovium, a layer of connective tissue that lines the joint capsule, and trigger a massive inflammatory response.

**Why Each Wrong Option is Incorrect**
**Option A:** While *Streptococcus pneumoniae* can cause septic arthritis, it is less common in adults and more typically seen in children.
**Option B:** *Neisseria gonorrhoeae* is a common cause of septic arthritis in sexually active young adults, but it is not the most common cause overall.
**Option C:** *Klebsiella pneumoniae* is a gram-negative bacterium that can cause septic arthritis, particularly in immunocompromised patients or those with underlying joint disease.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that septic arthritis is a medical emergency requiring prompt recognition and treatment to prevent joint destruction and long-term disability. A delay in diagnosis can lead to devastating consequences, including joint fusion and loss of function.

**Correct Answer: A. Staphylococcus aureus**