Psoas abscess is a purulent collection within the psoas muscle, usually spreading from spinal, urinary, or gastrointestinal infections. Clinical presentation is often insidious and easily missed:

1. “Flank” or lower-back pain
   Dull, persistent ache that radiates to the anterior hip or groin; movement, walking, or hip extension markedly worsens the pain.
2. Flexion contracture of the hip
   Involuntary flexion and external rotation relieve tension on the psoas sheath, so patients walk with a limp or cannot fully straighten the leg.
3. Palpable or visible mass
   A tender, sausage-shaped swelling may be felt below the inguinal ligament or in the lateral flank, especially if the abscess is large.
4. Fever and night sweats
   Low-grade or spiking pyrexia, chills, and drenching sweats are common; elderly or immunosuppressed patients may be afebrile.
5. Leukocytosis and raised inflammatory markers
   Blood tests usually show neutrophilia, elevated CRP and ESR; these indices help monitor treatment response.
6. Compressive neuropathy
   Extension into the iliac fossa can compress the femoral or obturator nerve, causing thigh numbness, weakness, or referred knee pain.
7. Systemic toxicity
   Persistent bacteraemia may lead to tachycardia, hypotension, and multi-organ dysfunction if drainage is delayed.

Because pain is referred to the hip, the diagnosis is often confused with arthritis, disc herniation, or appendicitis. Any patient with back/hip pain, flexed hip posture, and fever merits urgent spinal/abdomen CT or MRI to confirm the collection and guide percutaneous or surgical drainage.