**Core Concept**
Pyomyositis is a rare but serious condition characterized by the formation of abscesses within skeletal muscles. It is often caused by bacterial infections, particularly those involving the Streptococcus and Staphylococcus species.

**Why the Correct Answer is Right**
The correct answer is Staphylococcus aureus. This bacterium is a common cause of pyomyositis due to its ability to invade and infect muscle tissue. Staphylococcus aureus produces toxins and enzymes that facilitate its invasion and proliferation within the muscle cells, leading to the formation of abscesses. The bacteria also trigger an inflammatory response, which can lead to significant morbidity and mortality if left untreated.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Escherichia coli (E. coli) is a common cause of urinary tract infections, not pyomyositis.
* **Option B:** This option is incorrect because Klebsiella pneumoniae is a bacterium that can cause a variety of infections, including pneumonia and urinary tract infections, but it is not a common cause of pyomyositis.
* **Option D:** This option is incorrect because Pseudomonas aeruginosa is a bacterium that can cause a variety of infections, particularly in immunocompromised patients, but it is not a common cause of pyomyositis.

**Clinical Pearl / High-Yield Fact**
Staphylococcus aureus is a common cause of pyomyositis, particularly in immunocompromised patients or those with underlying medical conditions such as diabetes or HIV/AIDS.

**Correct Answer:** C. Staphylococcus aureus