A 5-month-old girl presents to the emergency department (ED) with a tender, enlarging, red-blue papule on her right upper arm. The lesion began as a red papule 2 days ago and has grown in size, with the surrounding area turning red and swollen. At the ED, the papule appears dark blue in the center. There is a history of exposure to a small brown spider on the day the papule first appeared, and the patient is discharged with the diagnosis of a possible spider bite.The following day, a second small, red papule appears on the patient's left anterior thigh, and she develops a fever. She returns to the ED and now is refusing to use her right arm. She is drinking well and has good urine output. There is no vomiting, but she did have diarrhea during the past week. The review of systems is otherwise normal.The child's past medical history is remarkable for a term, uncomplicated gestation and vaginal delivery and a 20-day hospitalization for group B streptococcal meningitis diagnosed at 2 weeks of age. There is no resulting neurologic deficit, and she remained healthy until last week. Immunizations are up to date, and she is receiving no medications. The family history is unremarkable.Physical examination reveals an alert, smiling child who does not move her right arm (Fig. 1). Her temperature is 100.0°F (37.8°C), respiratory rate is 32 breaths/ min, heart rate is 145 beats/min, and blood pressure is 80/40 mm Hg. There is an approximately 2.5-cm dark reddish-blue, fluid-filled bulla on the anterolateral right upper arm surrounded by an approximately 1-cm wide, flat, hemorrhagic halo (Fig. 2). Circumferential erythema and edema extend to the elbow and shoulder. The right upper arm is extremely tender to palpation. Right extremity capillary refill is brisk, and distal pulses are strong. Right elbow and shoulder range of motion are limited due to pain. She can move her right wrist and fingers without difficulty. A tender, 1-cm, red, firm nodule is present on the left anterolateral thigh, close to the groin (Fig. 3). Findings on examination of her head, eyes, ears, nose, and throat are normal. Auscultation of the chest reveals normal S1 and S2 heart sounds without a murmur and clear breath sounds. Abdominal and gen-*Pediatric resident. † Associate Professor, Pediatrics, St.
