Which of the following is NOT a vital sign?

The correct answer is skin color, as it is not classified as a vital sign. Vital signs are specific measurements that indicate the body's most basic functions and overall physiological status. These essential indicators include respiration rate, body temperature, heart rate, and blood pressure. Each of these vital signs provides critical information about a person's health, allowing healthcare providers to assess their well-being.

Skin color, while it can provide important clues about a patient's health—such as indicating oxygenation or circulation issues—is not standardized as a vital sign measurement. It is more of an observational aspect used in physical assessments rather than a quantifiable metric like the other options listed. Thus, while skin color can certainly inform clinical judgments, it does not meet the criteria to be classified as a vital sign.