A 28-year-old primigravida on postpartum day 3 presents with severe breast pain, swelling, and erythema of the left breast. She is exclusively breastfeeding. On examination, a tender, indurated area is palpable in the upper outer quadrant. What is the most common cause of this presentation?

Explanation

## Acute Breast Pathology in Lactation: Mastitis **Key Point:** The clinical triad of **localized breast erythema, induration/tenderness, and severe pain** in a breastfeeding woman is the hallmark presentation of **bacterial mastitis (puerperal mastitis)**, which is the most common cause of this specific constellation of findings. ### Why Mastitis (Option B) is Correct - Mastitis is defined as inflammation of the breast parenchyma, most commonly caused by *Staphylococcus aureus* (including MRSA strains) - It occurs in **2–10% of breastfeeding women** (Williams Obstetrics, 25th ed.) and is the **most common cause of focal, unilateral breast erythema + induration** in the postpartum period - Classic presentation: **unilateral**, focal erythema, warmth, induration, severe pain — often with fever >38.5°C and flu-like systemic symptoms - Onset is typically **postpartum days 3–21**, with a peak around days 5–10, but can begin as early as day 2–3 - The **upper outer quadrant** is the most commonly affected site (consistent with this case) ### Why Milk Stasis/Engorgement (Option A) is Incorrect Here - Engorgement is **bilateral and diffuse**, not focal or indurated - It lacks the **localized induration** and **erythema** described in this stem - Systemic symptoms are absent in pure engorgement - The stem explicitly describes a **tender, indurated area** — a hallmark of mastitis, not simple engorgement ### Comparison Table | Feature | Milk Stasis | Mastitis | Blocked Duct | Abscess | |---------|------------|----------|-------------|---------| | **Onset** | Days 2–5 PP | Days 3–21 PP | Variable | Days 10–21 PP | | **Fever** | Absent | High (>38.5°C) | Absent | High, persistent | | **Localization** | Bilateral, diffuse | Unilateral, focal | Focal, small | Localized, fluctuant | | **Induration** | Absent | Present | Mild | Present + fluctuance | | **Erythema** | Diffuse/mild | Focal, intense | Absent | Focal | | **Frequency** | Very common | Most common cause of focal findings | Common | Rare (<1%) | **Clinical Pearl:** Mastitis is treated with **continued breastfeeding** (or pumping), **dicloxacillin or cephalexin** (first-line antibiotics), warm compresses, and analgesics. Cessation of breastfeeding is NOT recommended as it worsens milk stasis and can promote abscess formation (Williams Obstetrics). **High-Yield:** If mastitis is inadequately treated, it can progress to **mammary abscess** (fluctuant mass requiring incision & drainage or needle aspiration). The key differentiator from abscess is the **absence of fluctuance** in mastitis.