**Core Concept**
Acute osteomyelitis is a bacterial infection of the bone, commonly caused by Staphylococcus aureus, particularly in children. The infection typically occurs due to hematogenous spread, where bacteria are carried from a distant site to the bone via the bloodstream.

**Why the Correct Answer is Right**
Staphylococcus aureus is the most common cause of acute osteomyelitis in children. This bacterium has a strong affinity for bone tissue and can adhere to the endosteal surface of the bone, leading to infection. The bacteria can enter the bone through the bloodstream, often from a distant site of infection such as the skin or respiratory tract. The proximal femur is a common site for osteomyelitis due to its rich blood supply.

**Why Each Wrong Option is Incorrect**
* **Option A:** Although Pseudomonas aeruginosa can cause osteomyelitis, it is more commonly associated with chronic or nosocomial infections, and less frequently seen in children.
* **Option B:** Escherichia coli is a common cause of urinary tract infections and some cases of osteomyelitis, but it is less likely to cause acute osteomyelitis in children compared to Staphylococcus aureus.
* **Option C:** Klebsiella pneumoniae can cause osteomyelitis, but it is more commonly seen in adults or as part of a polymicrobial infection.

**Clinical Pearl / High-Yield Fact**
Staphylococcus aureus is a common cause of osteomyelitis in children due to its ability to adhere to bone tissue and invade the bone through the bloodstream.

**Correct Answer:** D. Staphylococcus aureus