A 28-year-old primigravida presents to the postpartum clinic on day 18 after vaginal delivery with complaints of fever (38.8°C), myalgia, and breast pain. Examination reveals the wedge-shaped erythema marked **A** in the upper outer quadrant of the right breast, with warmth and tenderness. The patient has been exclusively breastfeeding. Which of the following organisms is most likely responsible for this clinical presentation?

Explanation

## Why Staphylococcus aureus (methicillin-sensitive) is right Lactational (puerperal) mastitis is caused by Staphylococcus aureus in >95% of cases, with methicillin-sensitive S. aureus (MSSA) being the most common strain in community-acquired infection. The clinical presentation—unilateral wedge-shaped erythema in the upper outer quadrant (marked **A**), fever >38.5°C, myalgia, and chills in a breastfeeding woman at 18 days postpartum—is pathognomonic for lactational mastitis. The bacteria typically enter through cracked nipple fissures and proliferate in areas of milk stasis. According to Williams Obstetrics 26e, S. aureus accounts for the overwhelming majority of lactational mastitis cases, and MSSA is the first-line consideration in non-healthcare-worker, community-acquired cases. ## Why each distractor is wrong - **Streptococcus agalactiae**: While GBS is a common cause of neonatal infection and can cause maternal bacteremia, it is NOT a primary pathogen in lactational mastitis. GBS mastitis is rare and does not account for >95% of cases. - **Escherichia coli**: E. coli may cause mastitis in non-lactating women or in cases of hematogenous spread from urinary tract infection, but it is not the predominant organism in lactational mastitis. It is a secondary consideration in recurrent or hospital-acquired cases. - **Candida albicans**: Candidal mastitis presents with deep, shooting pain after feeds and typically follows a course of antibiotics (which disrupts normal flora). It is not associated with acute fever, myalgia, and systemic symptoms as seen in this case. Candidal mastitis is a diagnosis of exclusion after bacterial causes are ruled out. **High-Yield:** Lactational mastitis = Staphylococcus aureus >95%; MSSA in community, MRSA in healthcare workers or recurrent cases; always continue breastfeeding and start antibiotics (dicloxacillin or cephalexin) for 10–14 days. [cite: Williams Obstetrics 26e — Lactational Mastitis, Pathogenesis and Microbiology]