A 32-year-old woman presents with a painless ulcer on her genitalia with indurated borders and a clean base. Dark-field microscopy of the exudate reveals motile, spiral-shaped organisms. What is the drug of choice for treatment of this infection?

Question

Accepted Answer

D. Penicillin G benzathine. ## Clinical Diagnosis

The presentation of a painless, indurated ulcer with a clean base and dark-field microscopy showing motile spirochetes is pathognomonic for **primary syphilis** caused by *Treponema pallidum*.

## Drug of Choice

**Key Point:** Penicillin G benzathine (2.4 million units IM as a single dose) is the first-line and gold-standard treatment for primary syphilis worldwide, including in India.

## Why Penicillin?

| Feature | Penicillin G Benzathine | Alternatives |
|---------|-------------------------|---------------|
| **Efficacy** | 95–98% cure rate | Slightly lower in some settings |
| **Mechanism** | β-lactam; inhibits cell wall synthesis | Same mechanism |
| **Dosing** | Single IM injection | Oral or IV regimens |
| **Cost** | Very low | Higher |
| **Guideline Status** | WHO, CDC, NACO first-line | Second-line |
| **Pregnancy** | Safe | Doxycycline contraindicated |

**High-Yield:** *Treponema pallidum* is a gram-negative spirochete with a thin peptidoglycan layer highly susceptible to penicillin. No resistance has been documented.

## Alternative Regimens

**Clinical Pearl:** In penicillin-allergic patients:
- **Non-pregnant:** Doxycycline 100 mg PO BD × 14 days (primary syphilis)
- **Pregnant:** Ceftriaxone 1 g IM daily × 10 days (β-lactam, safe in pregnancy)
- **Azithromycin:** Emerging resistance; no longer recommended as first-line [cite:CDC STI Guidelines 2021]

## Mnemonic

**PENICILLIN for Pallidum** — The organism name and drug start with the same letter; penicillin is the classic, irreplaceable choice.

Answer

A. Doxycycline

Answer

B. Azithromycin

Answer

C. Ceftriaxone

A 32-year-old woman presents with a painless ulcer on her genitalia with indurated borders and a clean base. Dark-field microscopy of the exudate reveals motile, spiral-shaped organisms. What is the drug of choice for treatment of this infection?

Explanation

Clinical Diagnosis

Drug of Choice

Why Penicillin?

Alternative Regimens

Mnemonic

Practice similar questions

Join our NEET PG community

Feature	Penicillin G Benzathine	Alternatives
Efficacy	95–98% cure rate	Slightly lower in some settings
Mechanism	β-lactam; inhibits cell wall synthesis	Same mechanism
Dosing	Single IM injection	Oral or IV regimens
Cost	Very low	Higher
Guideline Status	WHO, CDC, NACO first-line	Second-line
Pregnancy	Safe	Doxycycline contraindicated