**Core Concept**
Listeria monocytogenes is a gram-positive, intracellular bacterium that causes meningitis, particularly in immunocompromised patients, neonates, and the elderly. The treatment of choice is ampicillin due to its ability to penetrate the blood-brain barrier and effectively target intracellular bacterial growth.

**Why the Correct Answer is Right**
Ampicillin is the first-line agent for listeria meningitis because it is actively taken up by brain tissue and has potent activity against *Listeria monocytogenes*. It works by inhibiting bacterial cell wall synthesis via penicillin-binding proteins. The drug achieves reliable concentrations in the cerebrospinal fluid (CSF), which is essential for treating CNS infections. In combination with gentamicin (especially in severe cases), it significantly improves outcomes.

**Why Each Wrong Option is Incorrect**
Option B: Cefotaxime is a third-generation cephalosporin with good CSF penetration but is not active against *Listeria*. It is used for other bacterial meningitides (e.g., *Neisseria meningitidis*), but not as first-line for listeria.
Option C: Cefotriaxone is also a third-generation cephalosporin with good CSF penetration but lacks reliable activity against *Listeria* and is not recommended as monotherapy.
Option D: Ciprofloxacin is a fluoroquinolone with poor penetration into the CNS and no proven efficacy against listeria; it is not used in listeria meningitis.

**Clinical Pearl / High-Yield Fact**
Always use **ampicillin** as first-line therapy for listeria meningitis — it is the only antibiotic with proven efficacy against *Listeria monocytogenes* and is essential in immunocompromised or high-risk patients.

✓ Correct Answer: A. Ampicillin