**Question:** Dose and duration of benzathine penicillin G for prophylaxis of rheumatic fever is:-

A. 1.2 million units intramuscularly every three weeks for a total of 12 doses
B. 1.2 million units intramuscularly every three weeks for a total of 14 doses
C. 1.2 million units intramuscularly every three weeks for a total of 20 doses
D. 1.2 million units intramuscularly every three weeks for a total of 21 doses

**Correct Answer:** D. 1.2 million units intramuscularly every three weeks for a total of 21 doses

**Core Concept:**
Benzathine penicillin G is a long-acting form of penicillin used for the prophylaxis of rheumatic fever, which is a complication of untreated streptococcal infections. The drug works by inhibiting the growth of group A beta-hemolytic streptococci, preventing the development of rheumatic fever.

**Why the Correct Answer is Right:**
The correct answer is D. 1.2 million units intramuscularly every three weeks for a total of 21 doses. The drug is administered intramuscularly to ensure adequate penetration into the bloodstream to cover both the respiratory and gastrointestinal tracts, where streptococcal infections may occur. The total number of doses should be 21 (7 weeks) to ensure that a minimum of 14 doses are received within the first 21 days post-infection, which is the critical period for preventing the development of rheumatic fever.

**Why Each Wrong Option is Incorrect:**

A) 1.2 million units intramuscularly every three weeks for a total of 14 doses: This option is incorrect because it falls short of the required total number of doses (21) and the critical period (first 21 days post-infection).

B) 1.2 million units intramuscularly every three weeks for a total of 14 doses: This option is similar to A, with the same deficiencies in total doses and critical period.

C) 1.2 million units intramuscularly every three weeks for a total of 20 doses: This option is incorrect as it exceeds the required total number of doses (21) without providing any additional benefits. Owing to the short half-life of benzathine penicillin G, administering an extra dose will not significantly increase the drug's effectiveness.

**Clinical Pearls:**

1. Rheumatic fever is a serious complication of group A streptococcal infections that can lead to joint and heart damage.
2. Benzathine penicillin G is the first-line treatment for prophylaxis of rheumatic fever.
3. In high-risk areas, it is recommended to start prophylaxis within 48-72 hours of the onset of a streptococcal infection (pharyngitis, impetigo, etc.) and continue for 14 days.
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