**Core Concept**
Non-gonococcal septic arthritis is a medical emergency characterized by the infection of a joint space. The most common causative organisms are typically Gram-positive cocci, which can lead to significant joint damage and morbidity if not promptly treated.

**Why the Correct Answer is Right**
The most common cause of non-gonococcal septic arthritis is Staphylococcus aureus. This bacterium enters the joint space through a hematogenous route, often originating from a skin or soft tissue infection. The virulence factors of S. aureus, such as its ability to produce coagulase and form biofilms, contribute to its ability to cause severe joint infections. The joint space is particularly susceptible to infection due to its high concentration of synovial fluid, which can become a culture medium for pathogens.

**Why Each Wrong Option is Incorrect**
* **Option A:** Streptococcus pneumoniae is a common cause of pneumonia and meningitis but is less frequently associated with septic arthritis.
* **Option B:** Escherichia coli is a common cause of urinary tract infections and bacteremia but is not the most common cause of non-gonococcal septic arthritis.
* **Option D:** Klebsiella pneumoniae can cause various infections, including pneumonia and urinary tract infections, but is not typically associated with septic arthritis.

**Clinical Pearl / High-Yield Fact**
Staphylococcus aureus is the most common cause of non-gonococcal septic arthritis, and early diagnosis and treatment are crucial to prevent joint damage and improve outcomes.

**Correct Answer: C. Staphylococcus aureus**