What disorder is characterized by an increased heart rate when a person stands upright?

Postural orthostatic tachycardia syndrome (POTS) is defined by an abnormal increase in heart rate that occurs when a person transitions from lying down to an upright position. This physiological response is particularly pronounced in individuals with POTS, as their autonomic nervous system does not effectively regulate blood flow and heart rate during such postural changes. In healthy individuals, an increase in heart rate upon standing is a normal response to maintain blood pressure and cerebral perfusion, but in POTS patients, this response is exaggerated, often leading to a heart rate increase of 30 beats per minute or more within 10 minutes of standing.

The other disorders listed do not specifically present with this characteristic when a person changes position. Bradycardia is a condition of slow heart rate and would not show an increase when standing. Atrial fibrillation involves irregular heart rhythms but does not specifically correlate with the change in heart rate associated with standing, and ventricular tachycardia is a serious, sustained fast heart rhythm typically occurring in those with heart disease. Thus, POTS is uniquely identified by the significant heart rate increase upon standing, making it the correct answer.