**Core Concept**
Tetanus is a bacterial infection caused by Clostridium tetani, characterized by muscle rigidity and spasms. Effective treatment requires antibiotics that can target the bacteria and prevent further toxin production.

**Why the Correct Answer is Right**
Penicillin is not the preferred antibiotic for treating tetanus because it has limited activity against Clostridium tetani. The bacteria produce a unique toxin that is resistant to penicillin's mechanism of action. Instead, antibiotics like metronidazole and doxycycline are used to treat tetanus by inhibiting bacterial protein synthesis and preventing the production of tetanospasmin toxin. Clindamycin is also used as an alternative for its bacteriostatic properties against anaerobic bacteria like C. tetani.

**Why Each Wrong Option is Incorrect**
**Option A:** Clindamycin is a correct option for treating tetanus, making it an incorrect choice for "A/E" (all except).
**Option B:** Doxycycline is a suitable antibiotic for treating tetanus, making it an incorrect choice for "A/E".
**Option C:** Metronidazole is an effective antibiotic for treating tetanus, making it an incorrect choice for "A/E".

**Clinical Pearl / High-Yield Fact**
For tetanus treatment, it's essential to use antibiotics that can penetrate the central nervous system and prevent further toxin production. Metronidazole and doxycycline are preferred over penicillin due to their efficacy against anaerobic bacteria and ability to cross the blood-brain barrier.

**✓ Correct Answer: D. Penicillin**