Mastitis is most commonly caused by which organism?

• A. E. coli
• B. Staphylococcus
• C. Streptococcus
• D. Pseudomonas

Answer: (B)

Mastitis in lactating individuals is most commonly caused by Staphylococcus aureus. This organism is a common skin and nasal flora that can enter the breast through cracked nipples or milk ducts, especially when milk stasis occurs. Once inside, it tends to cause localized infection and inflammation, and can sometimes form an abscess. Because Staphylococcus aureus is the typical culprit in community-acquired mastitis, treatments are chosen to cover this organism, and continued breastfeeding or milk expression helps relieve stasis and promote drainage. While other bacteria like E. coli, Streptococcus, or Pseudomonas can cause breast infections, they are less common in this context.