Which of the following is a vital sign that a medical assistant must monitor?

Heart rate is a vital sign that a medical assistant must monitor because it provides essential information about a patient's cardiovascular health and overall physiological status. Vital signs are measurements of the body's most basic functions and are critical for assessing the health of a patient. They typically include body temperature, respiratory rate, blood pressure, and heart rate. Monitoring heart rate helps identify any abnormalities, such as tachycardia or bradycardia, which may indicate underlying health issues that require further investigation or intervention.

While blood glucose level, weight, and allergic reaction history are important health indicators and can be relevant in specific contexts, they do not fall under the category of vital signs. Blood glucose levels are particularly monitored in patients with diabetes or those at risk for hypoglycemia or hyperglycemia. Weight can be important for assessing growth, diet, and health status but is not classified as a vital sign. Allergic reaction history is crucial for understanding patient safety and management but does not serve as a vital sign measurement. Therefore, heart rate is the correct choice as it is a direct measurement critical to patient monitoring in medical settings.