Which condition is characterized by low blood pressure occurring upon standing?

Orthostatic hypotension is a condition that occurs when there is a sudden drop in blood pressure when a person stands up from a sitting or lying position. This happens because the body fails to regulate blood pressure properly in response to the change in position, leading to symptoms such as dizziness, lightheadedness, and sometimes fainting.

The fundamental mechanism behind orthostatic hypotension involves the body's inability to promptly increase heart rate and constrict blood vessels to maintain blood pressure when standing. This condition is particularly common in older adults or individuals who may have certain medical conditions or are taking medications that affect blood pressure regulation.

In contrast, postural hypertension refers to a condition of increased blood pressure upon standing, which is not what is being described in this question. Neurogenic shock and hypovolemic shock are more severe and involve significant decreases in blood volume or disruptions in the nervous system, leading to other complex symptoms and complications beyond just low blood pressure upon standing. Thus, orthostatic hypotension is the correct term for this specific phenomenon.