A 3-month-old girl is brought to the emergency department by her mother for increased fussiness over the last 2 days. The mother says, "She is fussier than usual, especially when I change her diaper or pick her up. I'm exhausted and cannot get her to stop crying." The infant has had no fever, runny nose, vomiting, or diarrhea. She breastfeeds every few hours but "less than usual" and has had 2 wet diapers in the last 24 hours. The patient was born via cesarean delivery for breech presentation at 34 weeks gestation. She did not require mechanical ventilation, and her neonatal intensive care unit course was unremarkable. The patient has been "a bit colicky" but is otherwise healthy; she received a set of vaccinations at age 2 months. Temperature is 37.2 C (99 F), pulse is 152/min, and respirations are 40/min. The patient cries throughout the examination and is difficult to console. Cardiopulmonary examination is unremarkable. The abdomen is soft and nondistended. The left leg is flexed and externally rotated and is held very still compared with the right. There is decreased range of motion of the left hip. Laboratory results show a leukocyte count of 16,000/mm³ and a C-reactive protein of 8.5 mg/L. Ultrasound of the left hip shows a small effusion. Which of the following is the most likely diagnosis for this patient?

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Explanation: 11ec2b69_c1d6_e8a9_8f63_25026ecee9aa_MD0005_00 The infant's poor appetite, decreased range of motion of the left hip, and elevated inflammatory markers are concerning for septic arthritis of the hip. Septic arthritis is a joint (frequently hip or knee) infection most commonly caused by Staphylococcus aureus. It classically presents with an acute onset of fever, joint pain, swelling, and refusal to bear weight. Neonates and infants often have more subtle signs, such as irritability, poor feeding, and aversion to being held. Fever may be absent. On examination, the hip is usually flexed, abducted, and externally rotated to relieve pressure on the inflamed joint capsule. Pseudoparalysis (lack of movement) and swelling of the affected joint may be present. Septic arthritis requires prompt diagnosis. The white blood cell count, erythrocyte sedimentation rate, and C-reactive protein are usually elevated, and blood cultures may be positive; ultrasound demonstrates an effusion. Treatment includes surgical drainage of the joint and parenteral antibiotics. (Choice A) Developmental dysplasia of the hip is an abnormally developed, mechanically unstable hip joint. Breech presentation and female sex (as well as positive family history and excessively tight swaddling) are risk factors. Examination findings include positive Barlow and Ortolani tests with hip dislocation, limited hip abduction, and asymmetric gluteal/inguinal folds. Ultrasound would not show effusion. Barlow and Ortolani tests (Choice B) Legg-Calvé-Perthes disease, or idiopathic avascular necrosis of the hip, presents in children age 3-12 with insidious-onset hip pain and limp. Deformity of the femoral head is seen on radiograph and MRI. Inflammatory markers are not generally elevated. radiograph (Choice C) Nonaccidental trauma can present with a fussy, inconsolable infant but is not associated with a hip effusion. Abnormal bruising, multiple fractures, retinal hemorrhages, subdural hemorrhages, and skull fractures are suspicious for this condition. (Choice E) Transient synovitis can present as a hip effusion, usually in children age 3-8, often following a viral illness; it is rare in a 3-month-old. Patients typically appear well (are consolable) with normal or only mildly elevated inflammatory markers. Educational objective: Infants with septic arthritis have subtle findings such as irritability, poor feeding, aversion to being held, and pseudoparalysis. Although fever may be absent, elevated inflammatory markers are red flags for infection. References: Bone and joint infections in children: septic arthritis. (http://www.ncbi.nlm.nih.gov/pubmed/26189923) Neonatal hip septic arthritis: ultrasound should not influence decision to aspirate. (http://www.ncbi.nlm.nih.gov/pubmed/23960065)

A 3-Month-Old Girl Is Brought to the Emergency Department by Her