**Core Concept**
Benzyl penicillin is a first-line treatment for primary syphilis, and its dosage is crucial to ensure effective treatment and prevent resistance. The recommended dosage is typically administered parenterally due to its poor oral bioavailability.

**Why the Correct Answer is Right**
The correct dosage of benzyl penicillin for primary syphilis is 2.4 million units administered intramuscularly as a single dose. This high dose is necessary to achieve adequate tissue concentrations in the central nervous system and other tissues where Treponema pallidum resides. The benzyl penicillin is then metabolized by the enzyme penicillinase (also known as beta-lactamase), which is produced by some bacteria, but this is not a concern in the case of Treponema pallidum.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a lower dose of 1.2 million units would be insufficient to achieve therapeutic levels in the central nervous system.
**Option B:** This option is incorrect because benzyl penicillin is not typically administered orally due to its poor absorption.
**Option C:** This option is incorrect because the recommended dosage for primary syphilis does not involve multiple doses.

**Clinical Pearl / High-Yield Fact**
It's essential to note that benzyl penicillin is not effective against Treponema pallidum if the patient has a history of penicillin allergy. In such cases, alternative treatments like ceftriaxone or doxycycline may be considered.

**Correct Answer:** D. 2.4 million units (intramuscularly as a single dose)