What instrument is specifically calibrated for measuring blood pressure in millimeters of mercury?

The sphygmomanometer is the instrument specifically designed to measure blood pressure in millimeters of mercury (mmHg). This device typically consists of an inflatable cuff, which is wrapped around the patient's arm, and a pressure gauge that reflects the pressure exerted by blood against the walls of the arteries. As the cuff inflates, it temporarily stops the flow of blood, and as it deflates, the gauge measures the pressure at which blood flow resumes, allowing healthcare professionals to determine systolic and diastolic blood pressure readings.

In contrast, a barometer measures atmospheric pressure and is not designed for blood pressure assessment. A stethoscope is used primarily to listen to internal sounds of a patient's body, such as heartbeats or lung sounds, but it does not measure pressure itself. An electrocardiograph records the electrical activity of the heart and is not related to measuring blood pressure. Thus, the sphygmomanometer is the only instrument among the choices that directly measures blood pressure and provides readings in the mmHg unit.