Final answer:
A positive Ortolani test, which checks for developmental dysplasia of the hip in newborns, is indicated by an audible click or clunk when the nurse abducts the infant's legs, as the femoral head pops back into the acetabulum.
Step-by-step explanation:
A nurse performing the Ortolani test on a newborn is checking for developmental dysplasia of the hip (DDH). A positive Ortolani test is indicated by an audible click or a clunk as the hip reduces into the joint when the nurse abducts the infant's legs. This sound or sensation occurs because the head of the femur pops back into the acetabulum. The other options listed such as dorsiflexion then fanning, hypertonia and jitteriness, and an arched back and crying, are not associated with a positive Ortolani test. These may relate to other conditions or reflex responses, like neurological reflexes such as the Babinski sign, which is assessed using the plantar reflex and is characterized by dorsiflexion of the foot and extension/splaying of the toes in response to stimulation of the plantar surface. The Ortolani test, however, is specific to the examination of the hips.