**Question:** 28 year old female presented with syphilitic chancre. She was put on Inj. Benzathine penicillin G 2.4 million units i.m single dose regimen. After initial treatment, she was screened at 6 month interval. She was found to be still positive. Which of the following treatment regimen would you recommend:

A. Increase the dosage
B. Change to oral penicillin
C. Adding doxycycline
D. Extend the treatment duration

**Correct Answer:** C. Adding doxycycline

**Core Concept:** Syphilis is a sexually transmitted infection caused by Treponema pallidum. The syphilitic chancre is the primary symptom. Treatment often involves penicillin, which targets the bacteria, and doxycycline, which targets the host immune response.

**Why the Correct Answer is Right:** Doxycycline is added to the treatment as it has a synergistic effect with penicillin. Doxycycline inhibits the production of penicillinase, an enzyme which breaks down penicillin, thereby enhancing the effectiveness of penicillin in treating syphilis.

**Why Each Wrong Option is Incorrect:**

A. Increasing the dosage of Benzathine penicillin G is not recommended as the treatment already involves a high dose (2.4 million units) which is effective against the bacteria.

B. Changing to oral penicillin is not ideal as oral penicillin is less efficient compared to the injectable form used. Syphilis treatment depends on the administration method being effective, not just the drug itself.

C. Adding doxycycline (Option C) increases the effectiveness of penicillin by inhibiting penicillinase production, preventing penicillin degradation, and enhancing the bactericidal effect.

D. Extending the treatment duration does not improve the outcome as the recommended duration for penicillin therapy is 14-21 days, and adding more days won't change its effectiveness.

**Clinical Pearl:** The addition of doxycycline to penicillin treatment is crucial for the successful management of syphilis, ensuring the inhibition of penicillinase production and enhancing the bactericidal effect of penicillin. The other options do not improve treatment efficacy.