**Core Concept**
The antibiotic of choice for endocarditis prophylaxis in high-risk patients undergoing dental procedures is based on the patient's allergy profile and pharmacokinetic suitability. In patients with penicillin allergy, macrolides or clindamycin are preferred, with clindamycin being the first-line agent due to its proven efficacy and safety in such scenarios.

**Why the Correct Answer is Right**
Clindamycin is the recommended alternative for patients with penicillin allergy who cannot take oral antibiotics. It is effective against Gram-positive organisms and covers *Streptococcus viridans*, the primary pathogen in infective endocarditis. It can be administered intravenously and is given 30 minutes before dental procedures to ensure adequate tissue concentration. Unlike macrolides, clindamycin has better coverage and is not associated with severe hypersensitivity reactions in penicillin-allergic patients.

**Why Each Wrong Option is Incorrect**
Option A: Clarithromycin is a macrolide and may be used in penicillin allergy, but it has lower efficacy against *Streptococcus viridans* and is not as reliably effective as clindamycin. Also, it is less preferred in high-risk patients.
Option B: Cephalexin is a first-generation cephalosporin and is contraindicated in patients with penicillin allergy due to cross-reactivity.
Option C: Cefadroxil is also a cephalosporin and shares cross-reactivity with penicillins, making it unsuitable for penicillin-allergic patients.

**Clinical Pearl / High-Yield Fact**
For penicillin-allergic patients, **clindamycin** is the preferred intravenous agent for endocarditis prophylaxis before dental procedures. Avoid cephalosporins and macrolides due to variable efficacy or cross-reactivity.

✓ Correct Answer: D. Clindamycin 600 mg. 30 min. before dental procedures