**Core Concept:** Benzyl penicillin, also known as Procyn, is a penicillin antibiotic used to treat syphilis caused by Treponema pallidum. The drug exerts its action by inhibiting bacterial cell wall synthesis, leading to cell lysis and death. Primary syphilis is the first stage of syphilis infection, characterized by the appearance of a painless ulcer (chancre) at the site of infection.

**Why the Correct Answer is Right:** In the case of primary syphilis, benzyl penicillin is given as a single dose (4 million units) intramuscularly (IM) or intravenously (IV). This high dose is required due to the high infectivity of the disease and the presence of the bacterium in various tissues throughout the body. Additionally, the drug is administered intravenously or intramuscularly to ensure a rapid and uniform distribution of the antibiotic throughout the body.

**Why Each Wrong Option is Incorrect:**

A. **Incorrect dosage:** Option A suggests administering a lower dose (2 million units), which is inadequate for treating primary syphilis, as the recommended dose is 4 million units.
B. **Incorrect route:** Option B suggests giving benzyl penicillin orally, which is not a suitable method of administration for primary syphilis treatment. The drug is administered intramuscularly or intravenously for rapid and uniform distribution.
C. **Incorrect dosage:** Option C suggests administering a higher dose (6 million units), which is not necessary and could lead to adverse effects due to overdosing.
D. **Incorrect route:** Option D suggests giving benzyl penicillin rectally, which is not a suitable method of administration for primary syphilis treatment. The drug is administered intramuscularly or intravenously for rapid and uniform distribution.

**Clinical Pearl:** Penicillin-based therapy is the gold standard for treating syphilis, including primary syphilis. The choice of administration route and dosage depends on the severity of the disease stage and the patient's response to treatment. In primary syphilis, a single dose of 4 million units administered intravenously or intramuscularly is sufficient. Monitoring the patient's response to treatment is crucial to ensure successful eradication of Treponema pallidum, the causative agent of syphilis. Incomplete treatment can lead to latent infection, reactivation, or progression to more severe stages of syphilis.