**Core Concept**
The management of lactating mothers requires careful consideration of the potential risks and benefits of antibacterial antibiotics. This includes evaluating the safety of these medications for both the mother and the infant. The primary concern is the transfer of the antibiotic into breast milk and its potential impact on the infant's gut flora and overall health.

**Why the Correct Answer is Right**
The correct answer is **A. Cephalexin**. Cephalexin is a first-generation cephalosporin antibiotic that is excreted into breast milk. Although the levels are generally considered to be low, it is still recommended to exercise caution when prescribing cephalexin to lactating mothers, especially if the infant is premature or has a compromised immune system. The American Academy of Pediatrics (AAP) has classified cephalexin as a medication that can be used during breastfeeding, but with caution.

**Why Each Wrong Option is Incorrect**
* **Option B:** Ciprofloxacin is a fluoroquinolone antibiotic that is contraindicated in lactating mothers due to its potential to cause cartilage damage in the infant. Although the risk is low, the AAP recommends avoiding fluoroquinolones during breastfeeding.
* **Option C:** Amoxicillin is a penicillin antibiotic that is generally considered safe for use during breastfeeding. It is excreted into breast milk in small amounts and is not expected to cause harm to the infant.
* **Option D:** Erythromycin is a macrolide antibiotic that is considered safe for use during breastfeeding. It is excreted into breast milk in small amounts and is not expected to cause harm to the infant.

**Clinical Pearl / High-Yield Fact**
When prescribing antibacterial antibiotics to lactating mothers, it is essential to consider the safety of the medication for both the mother and the infant. Always consult the AAP's list of medications that are safe to use during breastfeeding and exercise caution when prescribing medications that are not listed.

**Correct Answer:** A. Cephalexin.