**Core Concept**
Acute puerperal mastitis is a common postpartum infection of the breast, usually occurring within the first few days after delivery. The most common causative organism is a Gram-positive bacterium that thrives in the lactational environment of the breast.

**Why the Correct Answer is Right**
The correct organism is Staphylococcus aureus, which is a common commensal bacterium on the skin and mucous membranes. During lactation, the breast provides a nutrient-rich environment that allows S. aureus to proliferate and cause infection. This bacterium produces enzymes such as coagulase, which helps it to evade the host immune response.

**Why Each Wrong Option is Incorrect**
**Option A:** Staphylococcus epidermidis is a coagulase-negative bacterium that is also a common commensal on the skin, but it is less virulent and less commonly associated with mastitis.
**Option B:** Escherichia coli is a Gram-negative bacterium that can cause mastitis, but it is less common than S. aureus, particularly in the postpartum period.
**Option C:** Klebsiella pneumoniae is a Gram-negative bacterium that can cause a range of infections, but it is not typically associated with mastitis.

**Clinical Pearl / High-Yield Fact**
Remember that mastitis often presents with symptoms such as breast pain, swelling, and fever, and may be accompanied by a purulent discharge from the nipple. Prompt treatment with antibiotics is essential to prevent complications such as abscess formation and sepsis.

**Correct Answer: D. Staphylococcus aureus**