To avoid injury, how far should a rectal thermometer be inserted in infants?

The appropriate depth for inserting a rectal thermometer in infants is 2.5 cm. This depth is recommended to ensure that the thermometer accurately measures the core body temperature without risking injury or discomfort to the infant. Inserting the thermometer too deeply could potentially cause rectal perforation or injury, which is why the depth of 2.5 cm is considered safe and effective for obtaining a correct reading in this age group.

Other depths suggested by the other answer choices either fall short of the necessary depth for accurate measurement or extend beyond what is considered safe, leading to increased risk for the child. Proper technique is crucial not only for obtaining an accurate temperature reading but also for ensuring the safety and comfort of the infant.