Which type of thermometer is never used for taking rectal temperatures?

The choice identifying a thermometer with a long, narrow tip that is color-coded blue is correct because blue typically indicates a thermometer designed specifically for oral use. Using such a thermometer rectally is inappropriate due to potential safety concerns and the design of the thermometer.

Rectal thermometers are generally designed with a shorter, rounder tip to facilitate safe and comfortable insertion. Additionally, they are often color-coded with a different scheme to prevent cross-use between oral and rectal measurements.

In contrast, digital thermometers, mercury thermometers, and infrared thermometers can all be utilized for rectal temperature readings, albeit with specific types of digital and mercury thermometers designed for that purpose. Infrared thermometers measure temperature without direct contact, so their use is not typically associated with rectal measurements, but they don’t have the same restriction as the thermometer described.