## **Core Concept**
Acute hematogenous osteomyelitis (AHO) is an infection of the bone that occurs through hematogenous spread, typically affecting the long bones in children. The condition requires prompt diagnosis and treatment to prevent complications such as bone necrosis and septic arthritis. The most common causative organism is a gram-positive coccus.

## **Why the Correct Answer is Right**
The correct answer, *Staphylococcus aureus*, is the most common cause of acute hematogenous osteomyelitis, responsible for approximately 80% of cases. This organism is a virulent pathogen that can colonize the skin and mucous membranes, from where it can easily enter the bloodstream and seed in the bone. *Staphylococcus aureus* produces a variety of virulence factors, including adhesins that facilitate its attachment to bone and cartilage.

## **Why Each Wrong Option is Incorrect**
* **Option A:** *Streptococcus pneumoniae* can cause osteomyelitis but is less common compared to *Staphylococcus aureus*, particularly in the context of AHO. It is more commonly associated with infections like pneumonia and meningitis.
* **Option B:** *Escherichia coli* is a gram-negative rod that can cause osteomyelitis, especially in certain contexts such as urinary tract infections leading to bacteremia. However, it is not the most common cause of AHO.
* **Option D:** *Pseudomonas aeruginosa* can cause osteomyelitis, particularly in the setting of puncture wounds through the footpad or in immunocompromised patients, but it is not the most common cause of AHO.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the presence of fever, pain, and swelling in a child, particularly around the knee or other long bones, should prompt consideration of AHO. Blood cultures and imaging studies (like MRI) are critical for early diagnosis. *Staphylococcus aureus* is the leading cause, and initial antibiotic therapy should cover this organism.

## **Correct Answer:** .