**Core Concept**
Enterococcal infections are caused by Gram-positive bacteria, Enterococcus species, which are commonly found in the human gut. Treatment often involves antibiotics that target cell wall synthesis, protein synthesis, or DNA replication.

**Why the Correct Answer is Right**
Cephalexin is a first-generation cephalosporin antibiotic that primarily targets Gram-positive bacteria by inhibiting cell wall synthesis. However, Enterococcus species have developed resistance to cephalosporins, making cephalexin ineffective against enterococcal infections. In contrast, vancomycin, linezolid, and teicoplanin are used to treat enterococcal infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Vancomycin is a glycopeptide antibiotic that inhibits cell wall synthesis in Gram-positive bacteria, including Enterococcus species, making it an effective treatment for enterococcal infections.
**Option B:** Linezolid is an oxazolidinone antibiotic that inhibits protein synthesis by binding to the 50S ribosomal subunit, making it effective against Gram-positive bacteria, including Enterococcus species.
**Option C:** Teicoplanin is a glycopeptide antibiotic similar to vancomycin, which inhibits cell wall synthesis in Gram-positive bacteria, including Enterococcus species, making it an effective treatment for enterococcal infections.

**Clinical Pearl / High-Yield Fact**
When treating enterococcal infections, it's essential to consider the local resistance patterns and the potential for cross-resistance among antibiotics. Vancomycin-resistant Enterococcus (VRE) is a significant concern in healthcare settings, and alternative therapies like linezolid or daptomycin may be necessary.

**✓ Correct Answer: D. Cephalexin Cephalexin is not used in enterococcal infections due to resistance and limited effectiveness.**