**Core Concept**
The question is testing the student's knowledge of postpartum mastitis, a common condition in lactating women. Postpartum mastitis is an inflammation of the breast tissue, often caused by bacterial infection. The ideal antibiotic for postpartum mastitis should be effective against Staphylococcus aureus, the most common causative organism.

**Why the Correct Answer is Right**
Dicloxacillin is a penicillinase-resistant penicillin, which means it is effective against beta-lactamase-producing Staphylococcus aureus. This is crucial because S. aureus produces beta-lactamase, an enzyme that inactivates many penicillins. By resisting beta-lactamase, dicloxacillin can effectively target and eliminate the bacteria causing postpartum mastitis. Additionally, dicloxacillin has good penetration into breast tissue, making it an ideal choice for treating mastitis in lactating women.

**Why Each Wrong Option is Incorrect**
**Option A:** Ampicillin is a broad-spectrum penicillin, but it is not effective against beta-lactamase-producing Staphylococcus aureus. It may even be inactivated by the enzyme, making it a poor choice for postpartum mastitis.

**Option C:** Ceftazidime is a cephalosporin antibiotic that is effective against a wide range of Gram-negative bacteria. However, it is not the ideal choice for postpartum mastitis, as Staphylococcus aureus is a Gram-positive organism. Ceftazidime may not be as effective against this pathogen.

**Option D:** Ciprofloxacin is a fluoroquinolone antibiotic that is effective against a wide range of bacteria, including some Gram-positive organisms. However, it is not typically used to treat postpartum mastitis, and its use in lactating women is generally avoided due to concerns about potential effects on the infant's gut flora.

**Clinical Pearl / High-Yield Fact**
When treating postpartum mastitis, it's essential to choose an antibiotic that is effective against Staphylococcus aureus and has good penetration into breast tissue. Dicloxacillin is a good choice because it resists beta-lactamase and has been shown to be effective in treating mastitis in lactating women.

**✓ Correct Answer: B. Dicloxacillin**