## **Core Concept**
Mastitis in infants refers to the inflammation of breast tissue, which can occur in newborns, often due to infection or blockage of the milk ducts. It is more common in neonatal girls but can also occur in boys. The condition is usually caused by bacterial infection.

## **Why the Correct Answer is Right**
The correct answer, Staphylococcus aureus, is a common cause of mastitis in infants. This bacterium is a frequent pathogen in skin and soft tissue infections, including mastitis. The infection often presents with signs of inflammation such as redness, swelling, and warmth of the breast tissue.

## **Why Each Wrong Option is Incorrect**
* **Option A:** While E. coli can cause a variety of infections, it is less commonly associated with mastitis in infants compared to Staphylococcus aureus.
* **Option B:** Streptococcus pneumoniae can cause infections in children, but it is not the most common cause of mastitis in infants.
* **Option D:** Pseudomonas aeruginosa can cause infections, particularly in immunocompromised individuals or those with specific exposures, but it is not typically the primary cause of mastitis in infants.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that neonatal mastitis is often caused by Staphylococcus aureus. Clinicians should have a high suspicion for this pathogen when evaluating infants with mastitis, which guides appropriate antibiotic therapy.

## **Correct Answer:** . Staphylococcus aureus