Which can cause sinus tachycardia?

Sinus tachycardia is defined as a fast heart rate that originates in the sinus node, typically exceeding 100 beats per minute. One of the primary causes of sinus tachycardia is hyperthyroidism, a condition in which the thyroid gland produces excessive amounts of thyroid hormones. These hormones increase the metabolic rate and stimulate the heart, leading to an elevated heart rate.

In hyperthyroidism, the excess thyroid hormones can lead to a heightened sympathetic nervous system response, which increases heart rate and may cause the heart to beat faster even at rest. This physiological response is important, as it highlights how an imbalance of hormones can directly impact cardiovascular function.

Regarding the other options, inactivity often leads to bradycardia or a slower heart rate due to a decrease in heart functioning; excessive hydration might lead to other complications but does not typically result in tachycardia; and low blood pressure could stimulate a reflex response to increase heart rate, but it does not directly cause sinus tachycardia in the way that hyperthyroidism does. Understanding these distinctions helps to clarify why hyperthyroidism is the clear cause of sinus tachycardia among the provided choices.