**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, characterized by the production of a toxin that leads to severe respiratory and cardiac complications. The key to treating diphtheria lies in neutralizing this toxin and addressing the underlying infection.

**Why the Correct Answer is Right**
The primary treatment for diphtheria involves administering diphtheria antitoxin, which is a specific antibody that binds to and neutralizes the diphtheria toxin. This helps to prevent further tissue damage and reduces the severity of the disease. Antitoxin is typically administered as soon as possible after diagnosis, ideally within 24-48 hours of symptom onset. The antitoxin works by binding to the toxin, preventing it from interacting with host cells and causing damage.

**Why Each Wrong Option is Incorrect**
**Option B:** Tetracycline is an antibiotic that may be used to treat secondary bacterial infections that can occur in conjunction with diphtheria, but it is not the primary treatment for the disease itself.
**Option C:** Erythromycin is another antibiotic that may be used to treat diphtheria, particularly in cases where the patient is allergic to penicillin or other antibiotics. However, it is not the most important element in treatment.
**Option D:** Penicillin is an antibiotic that may be used to treat the bacterial infection that causes diphtheria, but it does not neutralize the diphtheria toxin and therefore is not the primary treatment for the disease.

**Clinical Pearl / High-Yield Fact**
In cases of diphtheria, it is essential to administer antitoxin promptly to prevent long-term complications, including cardiac and respiratory damage. Delaying treatment can lead to significant morbidity and mortality.

**✓ Correct Answer: A. Antitoxin**