A 52-year-old man presents with a 10-day history of fever, chills, and progressive pain in his right flank and groin. He reports that the pain is relieved when he lies on his back with his right knee drawn up toward his chest, but it becomes excruciating when he tries to walk or straighten his leg. His medical history is significant for poorly controlled type 2 diabetes mellitus. His temperature is 38.9°C (102°F), and his other vital signs are within normal limits. On physical examination, there is deep tenderness on palpation of the right lower quadrant and right flank. Passive extension of the right hip while the patient is lying on his left side results in severe pain. Laboratory studies show a white blood cell count of 16,000/mm3 and an elevated erythrocyte sedimentation rate (ESR).
Which of the following is the most likely diagnosis?
The correct answer is:
B) Iliopsoas abscess
The patient’s presentation is highly characteristic of an iliopsoas abscess, a collection of pus in the iliopsoas muscle compartment. The psoas muscle originates from the T12–L5 vertebrae and inserts into the lesser trochanter of the femur. Because of its deep anatomical position and proximity to the vertebral column, kidneys, and bowel, it can be seeded by hematogenous spread (primary abscess) or by direct extension from adjacent structures like the bowel or spine (secondary abscess). The psoas sign, which is pain elicited by passive hip extension, is a classic physical exam finding because stretching the inflamed muscle causes significant distress. Patients often adopt a fetal position with the hip flexed to minimize tension on the muscle.
Answer choice A: Acute appendicitis, is incorrect. Acute appendicitis can sometimes produce a positive psoas sign if the appendix lies retrocecally, but the overall picture does not match. Appendicitis typically has a much more acute onset, beginning with periumbilical discomfort that migrates to the right lower quadrant. It also does not generally cause prolonged flank or groin pain relieved by hip flexion. The prolonged course and significant flank involvement make this diagnosis unlikely.
Answer choice C: Nephtolithiasis, is incorrect. Kidney stones produce sudden, severe, colicky flank pain that may radiate to the groin, but they do not cause pain triggered by hip movement. Classic nephrolithiasis is not associated with high fever or marked leukocytosis unless complicated by infection, in which case the presentation tends to be more acute and not relieved by hip flexion. The subacute duration and the psoas-specific pain response argue strongly against nephrolithiasis.
Answer choice D: Osteoarthritis of the hip, is incorrect. Osteoarthritis produces chronic, mechanical hip or groin pain that worsens with weight-bearing. It does not cause fever, chills, leukocytosis, or elevated ESR. Additionally, the hip pain of osteoarthritis is joint-related and not influenced by passive extension of the hip in the same way that psoas pathology is.
Answer choice E: Trochanteric bursitis, is incorrect. Trochanteric bursitis causes lateral hip pain, particularly tenderness directly over the greater trochanter. It does not produce deep abdominal or flank pain and is not associated with fevers or other systemic inflammatory signs. Pain typically worsens when lying on the affected side, is not relieved by hip flexion, and would not elicit a positive psoas sign.
Key Learning Point
An iliopsoas abscess is an infection of the psoas muscle that presents with a subacute onset of fever, flank or groin pain, and a positive psoas sign (pain on hip extension). It is often seen in patients with risk factors such as diabetes, intravenous drug use, or Crohn disease. Diagnosis is typically confirmed via CT scan with intravenous contrast, and management involves both long-term antibiotics and percutaneous or surgical drainage.
