USMLE Microbiology: STIs—Neisseria gonorrhoeae, Chlamydia, Syphilis, HSV

What This Is and Why It Matters for USMLE STIs: Neisseria gonorrhoeae, Chlamydia, Syphilis, HSV

This topic is high-yield for Step 1, Step 2 CK, and Step 3. It appears in basic science, clinical, and ethics/management contexts. Understanding the pathophysiology, clinical presentation, diagnostic approach, and treatment of these STIs is crucial for diagnosing and managing patients.

High-Yield Facts (What You Must Memorize)

• Neisseria gonorrhoeae:
  • Gram-negative diplococcus
  • Causes gonorrhea, a common STI
  • Resistant to penicillin and tetracycline
• Chlamydia:
  • Obligate intracellular bacterium
  • Causes chlamydia, a common STI
  • Often asymptomatic in women
• Syphilis:
  • Spirochete bacterium
  • Causes syphilis, a chronic STI
  • Can cause congenital syphilis if left untreated
• HSV:
  • Herpes simplex virus
  • Causes herpes simplex, a viral STI
  • Can cause neonatal herpes if transmitted to newborns

Clinical Pearls & Buzzwords

• "Kissing disease"-HSV
• "Rash, fever, and lymphadenopathy"-Syphilis
• "Discharge and dysuria"-Gonorrhea or Chlamydia
• "Genital ulcers"-HSV or Syphilis
• "Asymptomatic infection in women"-Chlamydia

Step-by-Step Clinical Reasoning

1. Identify the syndrome or presentation (e.g., genital ulcers, discharge, or rash).
2. Generate a differential (most likely and must-not-miss):
   • Gonorrhea
   • Chlamydia
   • Syphilis
   • HSV
3. Order appropriate initial tests:
   • Nucleic acid amplification test (NAAT) for gonorrhea and chlamydia
   • Rapid plasma reagin (RPR) or Venereal Disease Research Laboratory (VDRL) test for syphilis
   • HSV PCR or culture
4. Interpret results:
   • Positive NAAT for gonorrhea or chlamydia
   • Positive RPR or VDRL test for syphilis
   • Positive HSV PCR or culture
5. Initiate treatment and monitoring:
   • Antibiotics for gonorrhea and chlamydia
   • Penicillin for syphilis
   • Antiviral therapy for HSV

Common Mistakes & Exam Traps

• The mistake: Failing to consider the possibility of co-infection with multiple STIs.
• Why it happens: Rushing through the differential diagnosis or not considering the patient's symptoms and risk factors.
• How to avoid it: Take your time to generate a thorough differential diagnosis, and consider the patient's symptoms and risk factors.

• Exam board insight: The examiners will penalize you for not considering the possibility of co-infection.

• The mistake: Not ordering a confirmatory test for syphilis after a positive RPR or VDRL test.

• Why it happens: Misunderstanding the difference between the RPR and VDRL tests.
• How to avoid it: Understand the difference between the RPR and VDRL tests, and order a confirmatory test (e.g., treponemal test) after a positive RPR or VDRL test.

• Exam board insight: The examiners will penalize you for not ordering a confirmatory test.

• The mistake: Not treating the patient's partner(s) for gonorrhea or chlamydia.

• Why it happens: Not considering the patient's risk factors and exposure history.
• How to avoid it: Consider the patient's risk factors and exposure history, and treat the patient's partner(s) if necessary.
• Exam board insight: The examiners will penalize you for not treating the patient's partner(s).

How It’s Tested on USMLE

• Step 1: Basic science vignette (e.g., molecular mechanism, pathology slide, pharmacology).
• Step 2 CK: Clinical vignette (e.g., "A 25-year-old with genital ulcers...").
• Step 3: Similar to Step 2 CK, plus prognosis, risk factors, and occasionally CCS management.

CCS (Step 3) Relevance (If Applicable)

• Initial orders: Order a NAAT for gonorrhea and chlamydia, and a RPR or VDRL test for syphilis.
• Monitoring and follow-up: Monitor the patient's symptoms and laboratory results, and follow up with the patient's partner(s) if necessary.
• Common mistakes: Not ordering a confirmatory test for syphilis after a positive RPR or VDRL test, and not treating the patient's partner(s) for gonorrhea or chlamydia.

Practice Questions (3-5 single-best-answer)

Question 1: A 25-year-old woman presents with genital ulcers and discharge. Which of the following is the most likely diagnosis?

A) Gonorrhea B) Chlamydia C) Syphilis D) HSV

Answer: C) Syphilis

Explanation: The patient's symptoms and laboratory results are consistent with syphilis. Gonorrhea and chlamydia typically cause discharge and dysuria, while HSV causes genital ulcers.

Question 2: A 30-year-old man presents with a rash and fever. Which of the following is the most likely diagnosis?

A) Gonorrhea B) Chlamydia C) Syphilis D) HSV

Answer: C) Syphilis

Explanation: The patient's symptoms and laboratory results are consistent with syphilis. Gonorrhea and chlamydia typically cause discharge and dysuria, while HSV causes genital ulcers.

Question 3: A 20-year-old woman presents with asymptomatic infection. Which of the following is the most likely diagnosis?

A) Gonorrhea B) Chlamydia C) Syphilis D) HSV

Answer: B) Chlamydia

Explanation: Chlamydia is often asymptomatic in women. Gonorrhea and syphilis typically cause symptoms, while HSV causes genital ulcers.

Quick Reference Card (60-Second Summary)

• STIs: Gonorrhea, Chlamydia, Syphilis, HSV
• First-line treatment: Antibiotics for gonorrhea and chlamydia, penicillin for syphilis, antiviral therapy for HSV
• Must-remember lab values: NAAT for gonorrhea and chlamydia, RPR or VDRL test for syphilis, HSV PCR or culture
• Mnemonic: "Kissing disease"-HSV

If You Get Stuck on Test Day

• Eliminate obviously wrong answers.
• Use the "next best step" hierarchy (least invasive, most specific).
• For Step 3 CCS: order basic labs, vitals, and IV access when unsure.

Related USMLE Topics

• Genital ulcers: Connects to HSV and Syphilis.
• Discharge and dysuria: Connects to Gonorrhea and Chlamydia.
• Asymptomatic infection: Connects to Chlamydia.