Seven-year-old boy who came to the emergency department with pain in the right hip, limping and fever. The symptoms had started abruptly seven days earlier. On the second day of illness he was assessed in the Traumatology Department and diagnosed with "hip synovitis", and prescribed rest and analgesics. The mother reported that during the following days the pain persisted at rest, woke him up at night and improved, but did not disappear, with analgesia. She presented at all times with antalgic postures, functional impotence and lameness. Fever persisted for seven days (maximum 38.5°C), accompanied by decreased appetite. In the last few days, two skin lesions appeared, one on the buttock and the other on the scalp.
On general inspection, we found a child in a wheelchair in fair general condition, pale and in an antalgic position with complete flexion of the right hip and knee. During the physical examination, the absence of swelling, pain or limitation at any joint level was surprising, and no bony tender points were found. Active mobilisation of the hip was reduced, while passive mobilisation was normal. The gait was very cautious, and in the standing position he did not bear weight on the right leg. Pediculosis infestation and a nodular, inflamed lesion, both on the scalp, were also observed.
Blood tests showed mild neutrophilia and increased acute phase reactants: fibrinogen 1008 mg/dl, C-reactive protein 173 mg/l and erythrocyte sedimentation rate of 100 mm/hour.
Ultrasound of the hip and abdominopelvic X-ray were normal, except for an antalgic position in the latter.

During his stay in the ED, and after administering intravenous analgesia, abdominal pain appeared progressively on deep palpation in the hypogastrium and right iliac fossa without defence or other signs of peritoneal irritation. An abdominal ultrasound was performed, which was normal, and acute surgical pathology was ruled out.
The study for fever without focus was completed with a chest X-ray and a urine test strip, which were negative, and it was decided to admit the patient to the Infectious Diseases Unit. The Paediatric Rheumatology Service was consulted and recommended a bone scan, which was carried out in the following days, with negative results.
He was admitted with treatment with intravenous amoxicillin-clavulanic acid. Given the normality of the tests performed and the clinical suspicion of psoas abscess, an abdominal magnetic resonance imaging (MRI) scan was requested, which showed an abscess in the right psoas muscle and osteomyelitis of the L4 vertebral body. The blood culture was sterile, but the puncture of the scalp abscess isolated methicillin-resistant Staphylococcus aureus, so antibiotherapy was modified to cover this germ, with meropenem and linezolid. The patient evolved favourably, without the need for surgical drainage.