Which of the following is least likely to cause electrical interference in the operating room?

The pneumatic drill is least likely to cause electrical interference in the operating room because it operates using compressed air rather than electricity. Electrical interference typically arises from devices that generate electromagnetic fields or operate using electrical currents, which can disrupt the functioning of sensitive monitoring equipment used during surgeries.

In contrast, electric saws, electrocautery devices, and ultrasound equipment all use electrical power to function, making them potential sources of electrical noise. Electric saws can produce interference due to their motors, and electrocautery devices are particularly notorious for introducing electrical noise and artifacts in neurophysiologic monitoring due to high-frequency currents used for coagulation and cutting tissue. Ultrasound equipment, while primarily used for imaging, also relies on electrical signals that can interfere with other electronic devices.

Thus, the pneumatic drill stands out as the least likely source of electrical interference due to its non-electrical operational mechanism.