**Core Concept**
In septic arthritis, the synovial fluid is characterized by an inflammatory response due to infection, leading to changes in its composition and appearance.

**Why the Correct Answer is Right**
The correct answer is C. Markedly increased polymorphonuclear leukocytes. This is because in septic arthritis, the synovial fluid is invaded by a large number of polymorphonuclear leukocytes (PMNs), also known as neutrophils, which are a type of white blood cell that plays a key role in the inflammatory response. The presence of a high number of PMNs in the synovial fluid is a hallmark of septic arthritis, indicating an acute inflammatory response to the infection. This is due to the release of chemotactic factors, such as leukotriene B4 and interleukin-8, which attract PMNs to the site of infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Clear color is incorrect because in septic arthritis, the synovial fluid is typically cloudy or turbid due to the presence of PMNs and other inflammatory cells.

**Option B:** High viscosity is incorrect because septic arthritis is characterized by a decrease in synovial fluid viscosity, not an increase. This is due to the breakdown of hyaluronic acid and other components of the synovial fluid.

**Clinical Pearl / High-Yield Fact**
The presence of a high number of PMNs in the synovial fluid is a critical diagnostic clue for septic arthritis, and a synovial fluid analysis should always include a cell count and differential to evaluate for this condition.

**Correct Answer Line**
✓ Correct Answer: C. Markedly increased polymorphonuclear leukocytes