**Core Concept**
Mastitis in infants is a condition characterized by inflammation of the breast tissue, often caused by an infectious agent, leading to pain, swelling, and fever. This condition typically occurs in the first few weeks of life, particularly in newborns who are breastfed. The underlying pathophysiology involves an immune response to an invading microorganism, which triggers an inflammatory cascade.

**Why the Correct Answer is Right**
The most common causative agent of mastitis in infants is Staphylococcus aureus, a Gram-positive bacterium that infects the breast tissue through the milk ducts. The bacteria trigger an inflammatory response, leading to the release of various cytokines and chemokines, which recruit white blood cells to the affected area. The resulting inflammation causes pain, swelling, and tenderness in the affected breast. In severe cases, mastitis can lead to abscess formation, which may require drainage.

**Why Each Wrong Option is Incorrect**
**Option A:** E. coli is a common cause of urinary tract infections, but it is not typically associated with mastitis in infants.
**Option B:** Group B streptococcus is a common cause of neonatal sepsis and pneumonia, but it is not the primary cause of mastitis in infants.
**Option C:** Candida albicans is a common cause of thrush in infants, but it is not typically associated with mastitis.

**Clinical Pearl / High-Yield Fact**
Mastitis in infants can be prevented by proper hand hygiene and breast care, including frequent expression of milk and thorough cleaning of the nipple and areola.

**Correct Answer: C. Staphylococcus aureus**