An increased heart rate may be caused by which of the following?

An increased heart rate, or tachycardia, is often a physiological response to stimuli that activates the body's sympathetic nervous system, which prepares the body for a "fight or flight" reaction. Painful stimuli are well-known to trigger this response, as the body interprets pain as a potential threat, leading to an increase in heart rate as part of an overall stress response. This mechanism helps ensure that blood and oxygen flow to vital organs and muscles in times of distress.

In the case of calm stimuli, these typically induce relaxation and decreased anxiety, which would not generally lead to an increased heart rate. Medication effects can vary significantly depending on the type of medication; some may decrease the heart rate while others can increase it, but this is not a consistent factor across all medications. Fluid overload typically pertains to changes in blood pressure and can lead to heart complications but does not directly cause a heart rate increase as a primary response. Therefore, the most reliable cause of an increased heart rate among the options provided is indeed painful stimuli.