USMLE Step 2 CK: Infectious Disease—STIs, PID, Syphilis, Gonorrhoea, Chlamydia, HIV Testing and Follow-up

What This Is and Why It Matters for USMLE

Sexually Transmitted Infections (STIs): PID, Syphilis, Gonorrhoea/Chlamydia, HIV Testing and Follow-up is a high-yield topic for Step 1, Step 2 CK, and Step 3. It appears in both basic science and clinical contexts, with a focus on diagnosis, treatment, and management.

High-Yield Facts (What You Must Memorize)

• Pathophysiology:
  • PID: ascending infection of the female reproductive tract, often due to Gonorrhoea or Chlamydia.
  • Syphilis: caused by Treponema pallidum, spread through sexual contact.
  • Gonorrhoea: caused by Neisseria gonorrhoeae, often asymptomatic in women.
  • Chlamydia: caused by Chlamydia trachomatis, often asymptomatic in women.
• Classic presentation and physical exam findings:
  • PID: lower abdominal pain, cervical motion tenderness, and adnexal tenderness.
  • Syphilis: primary stage: painless chancre at the site of infection; secondary stage: rash, fever, and lymphadenopathy.
  • Gonorrhoea: often asymptomatic in women, but may present with dysuria and purulent discharge.
  • Chlamydia: often asymptomatic in women, but may present with dysuria and abnormal cervical discharge.
• Diagnostic approach:
  • PID: Endocervical swab for Gonorrhoea and Chlamydia; Ultrasound for tubo-ovarian abscess.
  • Syphilis: Rapid plasma reagin (RPR) and Venereal Disease Research Laboratory (VDRL) tests; Treponema pallidum immobilization (TPI) test.
  • Gonorrhoea: Gram stain and culture of urethral discharge.
  • Chlamydia: Endocervical swab for Chlamydia trachomatis.
• First-line treatment and management:
  • PID: Ceftriaxone and Doxycycline; Metronidazole for anaerobic coverage.
  • Syphilis: Penicillin G for primary, secondary, and latent syphilis.
  • Gonorrhoea: Ceftriaxone and Azithromycin.
  • Chlamydia: Azithromycin or Doxycycline.
• Red flags, complications, and follow-up:
  • PID: Tubo-ovarian abscess, Infertility, and Ectopic pregnancy.
  • Syphilis: Neurosyphilis, Congenital syphilis, and Cardiovascular syphilis.
  • Gonorrhoea: Resistant strains, Disseminated gonococcal infection, and Neonatal gonococcal infection.
  • Chlamydia: Pelvic inflammatory disease, Ectopic pregnancy, and Infertility.

Clinical Pearls & Buzzwords

• "Chlamydia trachomatis"-Chlamydia.
• "Neisseria gonorrhoeae"-Gonorrhoea.
• "Treponema pallidum"-Syphilis.
• "PID"-Pelvic Inflammatory Disease.
• "STI"-Sexually Transmitted Infection.

Step-by-Step Clinical Reasoning

1. Identify the syndrome or presentation: PID, Syphilis, Gonorrhoea, or Chlamydia.
2. Generate a differential: Most likely and must-not-miss diagnoses.
3. Order appropriate initial tests: Endocervical swab for Gonorrhoea and Chlamydia; Ultrasound for tubo-ovarian abscess.
4. Interpret results: Confirm or rule out the suspected diagnosis.
5. Initiate treatment and monitoring: Start antibiotics and monitor for complications.

Missing a life-threatening complication, such as Neurosyphilis or Disseminated gonococcal infection, can lead to serious consequences.

Common Mistakes & Exam Traps

• The mistake: Failing to consider Resistant strains of Gonorrhoea.
• Why it happens: Misunderstanding or rushing through the question.
• How to avoid it: Take your time and consider all possible causes.
• Exam board insight: The examiners penalize this mistake by awarding few points.
• The mistake: Not ordering Ultrasound for tubo-ovarian abscess in PID.
• Why it happens: Misunderstanding or misreading the question.
• How to avoid it: Carefully read the question and consider all possible scenarios.
• Exam board insight: The examiners expect you to consider all possible complications.
• The mistake: Failing to treat Neurosyphilis.
• Why it happens: Misunderstanding or misreading the question.
• How to avoid it: Carefully read the question and consider all possible scenarios.
• Exam board insight: The examiners penalize this mistake by awarding few points.

How It’s Tested on USMLE

• Step 1: Basic science vignette, such as a molecular mechanism or pathology slide.
• Step 2 CK: Clinical vignette, such as a patient presentation or physical exam.
• Step 3: Similar to Step 2 CK, with a focus on diagnosis, treatment, and management.

Note common distractors and NBME tricks, such as:

• Asymptomatic patients with Gonorrhoea or Chlamydia.
• Resistant strains of Gonorrhoea.
• Neurosyphilis or Disseminated gonococcal infection.

CCS (Step 3) Relevance (If Applicable)

• Initial orders: Order Endocervical swab for Gonorrhoea and Chlamydia; Ultrasound for tubo-ovarian abscess.
• Monitoring and follow-up: Monitor for complications, such as Neurosyphilis or Disseminated gonococcal infection.
• Common mistakes: Failing to consider Resistant strains of Gonorrhoea or Neurosyphilis.

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

Question 1: A 25-year-old woman presents with lower abdominal pain and cervical motion tenderness. Which of the following is the most likely diagnosis? A) Gonorrhoea B) Chlamydia C) PID D) Syphilis Answer: C) PID Explanation: The patient's symptoms, such as lower abdominal pain and cervical motion tenderness, are consistent with PID.

Question 2: A 30-year-old man presents with a painless chancre on his penis. Which of the following is the most likely diagnosis? A) Gonorrhoea B) Chlamydia C) Syphilis D) Herpes simplex Answer: C) Syphilis Explanation: The patient's symptoms, such as a painless chancre, are consistent with syphilis.

Question 3: A 20-year-old woman presents with dysuria and abnormal cervical discharge. Which of the following is the most likely diagnosis? A) Gonorrhoea B) Chlamydia C) PID D) Bacterial vaginosis Answer: B) Chlamydia Explanation: The patient's symptoms, such as dysuria and abnormal cervical discharge, are consistent with chlamydia.

Quick Reference Card (60-Second Summary)

• PID: Lower abdominal pain, cervical motion tenderness, and adnexal tenderness.
• Syphilis: Painless chancre, rash, fever, and lymphadenopathy.
• Gonorrhoea: Often asymptomatic in women, but may present with dysuria and purulent discharge.
• Chlamydia: Often asymptomatic in women, but may present with dysuria and abnormal cervical discharge.
• First-line treatment: Ceftriaxone and Doxycycline for PID; Penicillin G for syphilis; Ceftriaxone and Azithromycin for gonorrhoea; Azithromycin or Doxycycline for chlamydia.

If You Get Stuck on Test Day

• Eliminate obviously wrong answers: Use the process of elimination to narrow down the possible answers.
• Use the "next best step" hierarchy: Consider the least invasive and most specific test or treatment.
• For Step 3 CCS: Order basic labs, vitals, and IV access when unsure.

Related USMLE Topics

• Pelvic inflammatory disease connects to Infertility and Ectopic pregnancy.
• Syphilis connects to Neurosyphilis and Congenital syphilis.
• Gonorrhoea connects to Disseminated gonococcal infection and Neonatal gonococcal infection.