**Core Concept**
Treatment of infective endocarditis, especially in patients with a history of anaphylaxis to penicillin, requires careful selection of antibiotics to avoid cross-reactivity and ensure efficacy. The patient's medical history of anaphylactic reactions to penicillin G necessitates alternative antibiotic options that target Enterococci without cross-reacting with penicillin.

**Why the Correct Answer is Right**
The patient's history of anaphylaxis to penicillin G indicates a high risk of cross-reactivity with other penicillins, such as ampicillin. Therefore, the treatment should focus on antibiotics that are effective against Enterococci without cross-reacting with penicillin. Vancomycin is a suitable alternative, as it has a distinct mechanism of action that targets the cell wall of Gram-positive bacteria, including Enterococci, without cross-reacting with penicillin. Vancomycin's efficacy in treating Enterococcal endocarditis is well-established, making it an ideal choice for this patient.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so we can skip it.
**Option B:** While ampicillin-sulbactam may be effective against some Enterococcal strains, its use in patients with a history of anaphylaxis to penicillin G is contraindicated due to the risk of cross-reactivity.
**Option C:** Gentamicin may be used as an adjunctive therapy in combination with vancomycin for Enterococcal endocarditis, but as a standalone treatment, it is not sufficient to address the infection.
**Option D:** This option is not provided, so we can skip it.

**Clinical Pearl / High-Yield Fact**
When treating patients with a history of anaphylaxis to penicillin, it is essential to consider alternative antibiotics that target the causative organism without cross-reacting with penicillin. Vancomycin is a reliable option for treating Enterococcal endocarditis in such cases.

**Correct Answer:** C. Vancomycin.