**Core Concept**
Postural orthostatic tachycardia syndrome (POTS) is a disorder characterized by excessive heart rate increase in response to upright posture, leading to symptoms like dizziness, lightheadedness, and nausea. This condition is often associated with autonomic nervous system dysfunction, particularly affecting the sympathetic and parasympathetic branches.

**Why the Correct Answer is Right**
POTS is characterized by a sustained increase in heart rate (more than 30 beats per minute) within 10 minutes of standing, accompanied by symptoms such as palpitations, tremors, and gastrointestinal disturbances. This response is typically mediated by the sympathetic nervous system, which is responsible for increasing heart rate and blood pressure in response to postural changes. In POTS, there is often an imbalance between sympathetic and parasympathetic activity, leading to excessive sympathetic activation.

**Why Each Wrong Option is Incorrect**
**Option A:** Orthostatic hypotension is a hallmark feature of POTS, characterized by a rapid decrease in blood pressure upon standing, often leading to symptoms like dizziness and fainting. Therefore, option A is incorrect because it is actually a characteristic of POTS.

**Option B:** Symptoms of POTS can include headache, fatigue, and sleep disturbances, making option B incorrect as it is a common feature of the condition.

**Option C:** POTS can be caused by various factors, including genetic predisposition, autoimmune disorders, and physical or psychological stress, making option C incorrect as it is a potential underlying cause of POTS.

**Clinical Pearl / High-Yield Fact**
POTS patients often exhibit a delayed increase in blood pressure in response to the Valsalva maneuver, which can help differentiate this condition from other causes of orthostatic intolerance.

**Correct Answer: D.**