Step 1 topics → Reproductive → Epididymitis and Orchitis

Epididymitis and Orchitis

USMLE Step 1 trap: Applies STI-based treatment for epididymitis regardless of patient age. In men under 35, epididymitis is most commonly caused by Chlamydia trachomatis or N. gonorrhoeae; in men over 35, enteric gram-negative rods (E. coli) predominate.

Epididymitis and orchitis are inflammatory conditions of the male reproductive tract that the USMLE Step 1 tests primarily through clinical vignettes requiring you to match patient age and sexual history to the correct organism — and then to the correct treatment. The most common presentation is a young male with unilateral scrotal pain and swelling that comes on gradually (as opposed to the sudden onset of torsion). You need to know not just the organisms but why age matters in distinguishing them, and how to handle the torsion differential under exam pressure.

Mumps orchitis gets tested as a separate clinical scenario: a post-pubertal male who recently had parotitis develops testicular swelling and pain days later. The timing is the key detail — students frequently assume orchitis happens at the same time as the parotitis, but it's a delayed complication. USMLE Step 1 will exploit this by giving you a vignette where parotitis already resolved before testicular symptoms appear, making the connection less obvious.

The trickiest part of this topic is the Prehn sign. Many students memorize it as the way to differentiate epididymitis from torsion — but the exam tests whether you know it's clinically unreliable. If a vignette presents acute scrotal pain and asks what to do next, the answer is Doppler ultrasound (or surgical exploration), not clinical maneuvers. Anchoring to Prehn sign is exactly the kind of shortcut that gets punished on USMLE Step 1.

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Common misconceptions

Common mistake
Wrong: Chlamydia/Gonorrhea is the cause of epididymitis in all age groups.
Right: In men under 35, epididymitis is most commonly caused by Chlamydia trachomatis or N. gonorrhoeae; in men over 35, enteric gram-negative rods (E. coli) predominate.
Chlamydia and gonorrhea cause epididymitis in sexually active men under 35, but the same reflex doesn't apply to older men. In men over 35, the culprits shift to enteric gram-negative rods like E. coli — the same organisms that cause UTIs — often in the setting of urinary tract instrumentation or structural urologic abnormality. Applying doxycycline/ceftriaxone to an older man with epididymitis is the wrong call; fluoroquinolones covering gram-negatives are more appropriate. Always anchor your organism selection to the patient's age and risk factors.
Common mistake
Wrong: Mumps orchitis occurs simultaneously with parotitis.
Right: Mumps orchitis typically develops 4–8 days after parotitis onset, not concurrently.
Mumps orchitis is not simultaneous with parotitis — it's a post-infectious complication that typically appears 4–8 days after parotitis begins. This delay matters because a vignette may present a patient whose parotitis has already improved, making the connection to mumps less obvious if you expect both to occur together. Think of orchitis as a secondary wave, not part of the initial presentation.
Common mistake
Wrong: A positive Prehn sign (pain relief with scrotal elevation) rules out torsion and confirms epididymitis.
Right: Prehn sign is unreliable for ruling out torsion; testicular torsion is a clinical emergency that requires urgent Doppler ultrasound or surgical exploration regardless of Prehn sign.
Prehn sign (pain relief with scrotal elevation) was historically taught as a way to favor epididymitis over torsion, but it is not reliable enough to rule out torsion in clinical practice or on the exam. Testicular torsion is a surgical emergency where delayed management causes irreversible ischemia, so you cannot afford a false-negative clinical sign. The correct next step when torsion is in the differential is immediate Doppler ultrasound or surgical exploration — not a physical exam maneuver.
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What the exam tests

  1. Given a patient's age and sexual history, identify the most likely causative organism of epididymitis and select the appropriate antibiotic regimen — recognizing that STI pathogens dominate in men under 35 while enteric gram-negatives dominate in men over 35.
  2. Recognize mumps orchitis as a delayed complication of mumps parotitis, occurring 4–8 days after parotitis onset, and understand that bilateral orchitis carries a significant risk of infertility due to testicular atrophy.

Can you avoid these mistakes?

A 19-year-old sexually active male presents with 3 days of gradually worsening left-sided scrotal pain and dysuria. Urethral discharge is noted. What is the most likely organism, and what is the first-line treatment?
A 58-year-old man with a history of BPH and recent urinary catheterization presents with unilateral scrotal swelling, tenderness, and fever. What organism class is most likely responsible, and how does this differ from the same presentation in a 22-year-old?
A 17-year-old presents with right-sided scrotal pain. On exam, elevation of the scrotum temporarily relieves the pain (positive Prehn sign). What is your next step in management, and why?
A 16-year-old male had parotid gland swelling 6 days ago that is now resolving. Today he presents with right testicular pain and swelling. What is the diagnosis, what is the timing pattern, and what long-term complication should you counsel him about?

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