**Core Concept**
The development of antibiotic resistance in bacteria is a complex process involving multiple mechanisms, including genetic mutations and gene transfer. In the context of meningitis caused by gram-positive cocci, the most likely etiologic agent is *Streptococcus pneumoniae*.

**Why the Correct Answer is Right**
*Streptococcus pneumoniae* is a common cause of bacterial meningitis in children, and resistance to beta-lactam antibiotics is a growing concern. The primary mechanism of resistance in *S. pneumoniae* involves the production of beta-lactamase enzymes, which inactivate beta-lactam antibiotics by hydrolyzing their beta-lactam ring. The most well-known beta-lactamase enzyme in *S. pneumoniae* is penicillin-binding protein 2a (PBP2a), which is encoded by the *bla* gene.

**Why Each Wrong Option is Incorrect**
* **Option A:** While efflux pumps are a mechanism of resistance in some bacteria, they are not the primary mechanism of resistance in *S. pneumoniae* to beta-lactam antibiotics.
* **Option B:** Mutations in the target of the antibiotic, such as penicillin-binding proteins (PBPs), can also contribute to resistance. However, the primary mechanism in *S. pneumoniae* is the production of beta-lactamase enzymes.
* **Option C:** Altered target affinity is a mechanism of resistance in some bacteria, but it is not the primary mechanism in *S. pneumoniae*.

**Clinical Pearl / High-Yield Fact**
*Streptococcus pneumoniae* is a leading cause of bacterial meningitis in children, and resistance to beta-lactam antibiotics is a growing concern. Clinicians should be aware of the local prevalence of resistant strains and consider alternative antibiotics, such as vancomycin or ceftriaxone, in cases of suspected or confirmed meningitis.

**Correct Answer:** C.