Step 1 topics → Musculoskeletal, Skin, and Connective Tissue → Psoas Abscess

Psoas Abscess

Psoas abscess is a collection of pus within the iliopsoas muscle compartment, and while it's a low-yield topic on USMLE Step 1, it shows up in clinical vignettes that test your ability to recognize an unusual presentation of hip pain or back pain with systemic infection. The exam won't ask you to memorize obscure details — it tests whether you can identify the classic posture, link the right organism to the right mechanism, and know that antibiotics alone won't cut it. The tricky part is that the presentation mimics other conditions (appendicitis, hip pathology, vertebral osteomyelitis), so the exam rewards students who connect the dots across multiple findings rather than anchoring on one symptom.

USMLE Step 1 approaches this concept through clinical vignettes: a patient with fever, flank or back pain, and a characteristic hip posture, or a patient with Crohn disease or IV drug use who develops a new deep-seated infection. The passage will give you enough clues to distinguish primary from secondary psoas abscess and select the right management — but only if you have the right mental models going in. Students most commonly stumble by not recognizing the positive psoas sign as a physical exam finding, and by assuming S. aureus is the answer regardless of context.

The core framework: primary abscess = hematogenous spread = S. aureus (think IV drug users, immunocompromised). Secondary abscess = contiguous spread from adjacent structures = enteric organisms or TB (think Crohn disease, vertebral osteomyelitis, bowel perforation). Management always includes drainage — this is non-negotiable and a common trap on USMLE Step 1 for students who think antibiotics alone are sufficient for deep-space infections.

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Common misconceptions

Common mistake
Gap: Misses the hip flexion posture and positive psoas sign as key exam findings
Patients with psoas abscess often lie with the hip flexed to relieve tension on the inflamed psoas muscle, and passive hip extension reproduces pain (positive psoas sign).
The psoas muscle is an active hip flexor, so when it's inflamed, the patient instinctively flexes the hip to reduce tension and pain — this is a protective posture, not a random finding. Passive extension of the hip stretches the inflamed psoas, reproducing pain and constituting a positive psoas sign. If you miss this, you'll overlook the key physical exam clue that points you toward a retroperitoneal process rather than an intraabdominal or joint problem.
Common mistake
Wrong: Staphylococcus aureus is the predominant organism in secondary psoas abscess.
Right: Primary psoas abscess (hematogenous) is caused by S. aureus; secondary psoas abscess (from contiguous spread, e.g., Crohn disease, vertebral osteomyelitis) involves enteric organisms or Mycobacterium tuberculosis.
The distinction between primary and secondary psoas abscess isn't just academic — it directly determines your organism and antibiotic choice. Primary abscess results from bacteremic seeding of the muscle (most common in immunocompromised patients or IV drug users), and S. aureus is the dominant pathogen. Secondary abscess comes from direct extension of an adjacent infection — Crohn disease, diverticulitis, appendicitis, or vertebral osteomyelitis — so the organisms reflect the source: enteric gram-negatives, anaerobes, or M. tuberculosis in endemic regions. Defaulting to S. aureus for all psoas abscesses will cost you on vignettes that clearly establish a GI or spinal source.
Common mistake
Wrong: Antibiotics alone are sufficient to treat psoas abscess.
Right: Psoas abscess requires CT- or ultrasound-guided percutaneous drainage (or surgical drainage) in addition to antibiotics.
Antibiotics penetrate poorly into walled-off collections with necrotic debris, which is exactly what an abscess is — the physical barrier prevents adequate antibiotic delivery regardless of organism sensitivity. Drainage (preferably CT-guided percutaneous drainage, or surgical if not feasible) removes the infectious nidus and allows antibiotics to work on residual infection. Treating a psoas abscess with antibiotics alone leads to treatment failure, and the exam will test whether you recognize drainage as a required, not optional, component of management.
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What the exam tests

  1. Recognize the classic presentation: fever with flank or back pain plus a hip held in flexion — and know that passive hip extension reproduces pain (positive psoas sign), which distinguishes psoas abscess from other causes of hip pain.
  2. Identify the correct causative organism based on abscess type: S. aureus for primary (hematogenous) psoas abscess versus enteric gram-negatives or Mycobacterium tuberculosis for secondary abscess arising from contiguous spread (e.g., Crohn disease, vertebral osteomyelitis).
  3. Select appropriate management: CT or ultrasound-guided percutaneous drainage is required in addition to antibiotics — antibiotics alone are insufficient for treating a walled-off abscess in the psoas compartment.

Can you avoid these mistakes?

A 34-year-old man with a history of IV drug use presents with 2 weeks of fever, right-sided lower back pain, and a limp. On exam, his right hip is held in flexion and passive extension of the hip reproduces severe pain. What is the most likely diagnosis and the most likely causative organism?
A 28-year-old woman with known Crohn disease presents with left flank pain, fever, and inability to fully extend her left hip. CT scan confirms a psoas abscess. How does the likely causative organism differ from a psoas abscess in an otherwise healthy patient with bacteremia?
A patient is diagnosed with psoas abscess confirmed on CT scan. Blood cultures are pending. The intern suggests starting broad-spectrum antibiotics and observing. What is wrong with this plan, and what must be added to the management?
What anatomical reason explains why a patient with psoas abscess holds the hip in flexion, and what does passive hip extension test on physical examination?

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