Step 1 topics → Microbiology → Syphilis (Treponema pallidum)

Syphilis (Treponema pallidum)

USMLE Step 1 trap: Confuses the painless syphilitic chancre with the painful ulcers of HSV or chancroid. The primary syphilis chancre is classically painless and indurated, which distinguishes it from the painful ulcers of herpes simplex or chancroid.

Syphilis is caused by Treponema pallidum, a spirochete that cannot be cultured and is too thin to see on Gram stain — you visualize it with dark-field microscopy. USMLE Step 1 tests every stage of this disease, and the primary chancre is the most commonly confused finding: it is painless, not painful. That single feature separates it from herpes (painful vesicles) and chancroid (painful ulcer) — when a vignette describes a painless, indurated genital ulcer, syphilis tops the list. The exam also exploits the Jarisch-Herxheimer reaction, which students reflexively call a penicillin allergy when it's actually a cytokine storm from rapid spirochete killing — treatment should not be stopped. Expect questions on serology workflow, tertiary mechanisms (vasa vasorum endarteritis → ascending aortic aneurysm), and congenital infection.

What makes syphilis genuinely tricky is how many things about it run counter to intuition. The primary ulcer doesn't hurt — that trips up a ton of students who conflate it with herpes. The screening tests aren't confirmatory. The treatment reaction looks like an allergic reaction but isn't. Tertiary cardiovascular disease looks nothing like standard coronary artery disease. Each of these is an active misconception the exam exploits, not just a trivia gap. You have to know not just the right answer but why the wrong answer is wrong.

On USMLE Step 1, syphilis questions often embed the key finding in a longer passage — a pregnant patient's prenatal labs, a traveler with a painless genital ulcer, an elderly patient with new aortic regurgitation. The answer hinges on pattern recognition across stages and the ability to distinguish syphilis from look-alike diagnoses. Master the stage-by-stage progression, nail the serology sequence, and understand the mechanisms behind tertiary manifestations and you'll be well-positioned.

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Common misconceptions

Common mistake
Wrong: The primary syphilis chancre is painful, like a herpes ulcer.
Right: The primary syphilis chancre is classically painless and indurated, which distinguishes it from the painful ulcers of herpes simplex or chancroid.
The painless nature of the syphilitic chancre is diagnostically critical — it's what distinguishes it from herpes simplex ulcers and chancroid, both of which are painful. The chancre is also indurated (firm, raised border), clean-based, and usually solitary. When a vignette describes a painless genital ulcer, syphilis should immediately jump to the top of your differential; pain shifts you toward HSV or H. ducreyi.
Common mistake
Wrong: A positive RPR or VDRL is sufficient to confirm syphilis diagnosis.
Right: RPR and VDRL are non-treponemal (screening) tests that can be false-positive; a positive result must be confirmed with a treponemal test (FTA-ABS or TPPA).
RPR and VDRL detect antibodies against cardiolipin-cholesterol-lecithin antigen — not against T. pallidum itself — which is why they can be false-positive in conditions like lupus, pregnancy, or viral infections. A reactive RPR or VDRL must always be confirmed with a treponemal-specific test (FTA-ABS or TPPA) before you call it syphilis. Think of non-treponemal tests as the screening step and treponemal tests as the confirmatory step; they also serve different purposes — RPR/VDRL titers track treatment response, while treponemal tests remain positive for life regardless of cure.
Common mistake
Wrong: The Jarisch-Herxheimer reaction is an allergic reaction to penicillin requiring drug discontinuation.
Right: The Jarisch-Herxheimer reaction is a febrile inflammatory response to cytokine release from dying spirochetes within hours of treatment; it is not a penicillin allergy and does not require stopping therapy.
The Jarisch-Herxheimer reaction is caused by massive cytokine release (TNF-α, IL-6, IL-8) when penicillin rapidly kills large numbers of spirochetes, not by an immune response to the drug itself. It presents within hours of the first dose as fever, rigors, and worsening of lesions — alarming, but self-limited. Critically, it is not a penicillin allergy, and treatment should not be stopped. Managing it is symptomatic (antipyretics); confusing it with anaphylaxis and switching antibiotics is a dangerous and testable error.
Common mistake
Wrong: Tertiary syphilis causes atherosclerotic coronary artery disease.
Right: Tertiary syphilis causes obliterative endarteritis of the vasa vasorum, leading to aortic root dilation, aortic regurgitation, and aneurysm of the ascending aorta—not coronary atherosclerosis.
Syphilitic cardiovascular disease results from obliterative endarteritis of the vasa vasorum — the small vessels that supply the aortic wall — causing ischemia and destruction of the media of the ascending aorta. This leads to dilation of the aortic root, aortic regurgitation, and aneurysm of the ascending aorta (the part of the aorta most vulnerable because of its rich vasa vasorum supply). This is mechanistically entirely different from atherosclerotic coronary artery disease, which involves lipid deposition and plaque formation in epicardial vessels. When you see ascending aortic aneurysm plus aortic regurgitation on Step 1, think syphilitic aortitis.
Common mistake
Wrong: Congenital syphilis is transmitted during delivery, like neonatal HSV or GBS.
Right: Treponema pallidum crosses the placenta hematogenously, so congenital syphilis is acquired in utero (transplacentally), not during passage through the birth canal.
T. pallidum is a small spirochete capable of crossing the placenta hematogenously at any point in pregnancy, though risk is highest in secondary syphilis when spirochetemia is greatest. This is fundamentally different from pathogens like GBS or HSV-2 that infect the neonate during passage through an infected birth canal. The practical implication: a cesarean section does NOT prevent congenital syphilis the way it can reduce neonatal HSV risk. Treatment of the mother during pregnancy is the prevention strategy.
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What the exam tests

  1. Recognize the classic features of the primary syphilis chancre — painless, indurated, single ulcer — and know that dark-field microscopy of the lesion is the direct diagnostic method at this stage.
  2. Identify secondary syphilis by its timing (weeks after primary), the hallmark diffuse maculopapular rash that includes the palms and soles, and associated condylomata lata and mucous patches.
  3. Explain the mechanisms and clinical manifestations of tertiary syphilis, including Argyll-Robertson pupils, tabes dorsalis, gummas, and cardiovascular syphilis affecting the ascending aorta via vasa vasorum endarteritis.
  4. Describe the features of congenital syphilis — including saber shins, saddle-nose deformity, Hutchinson teeth, interstitial keratitis — and recognize that transmission is transplacental, not intrapartum.
  5. Navigate the syphilis serology workflow: non-treponemal tests (RPR, VDRL) for screening and monitoring, treponemal tests (FTA-ABS, TPPA) for confirmation, and the significance of the Jarisch-Herxheimer reaction after initiating penicillin G treatment.

Can you avoid these mistakes?

A 24-year-old man presents with a single, painless, indurated ulcer on the glans penis with rubbery inguinal lymphadenopathy. His RPR comes back reactive. What is the next step to confirm the diagnosis, and what would dark-field microscopy of the lesion show?
A patient with confirmed secondary syphilis is treated with intramuscular benzathine penicillin G. Two hours later she develops fever, chills, and headache. Her nurse calls you concerned about a penicillin reaction. How do you explain what is happening and what is the correct management?
A 58-year-old man is found to have a new aortic regurgitation murmur and imaging shows dilation of the ascending aorta. His FTA-ABS returns positive. What is the pathophysiologic mechanism linking T. pallidum infection to this finding, and why is this different from typical aortic disease?
A pregnant woman at 28 weeks is found to have a reactive VDRL on routine prenatal screening. She has no symptoms and denies any history of STIs. What additional test is required before treating her, and what features in her newborn would suggest congenital syphilis was not prevented?

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