Postural Orthostatic Tachycardia Syndrome (POTS): This is the most likely diagnosis given the question, as POTS is characterized by a rapid increase in heart rate and other symptoms that occur upon standing. The condition is often diagnosed based on the presence of these symptoms and an increase in heart rate of 30 beats per minute or more within 10 minutes of standing.

Dehydration: Dehydration can cause symptoms similar to POTS, including dizziness, lightheadedness, and tachycardia upon standing. It's a common and easily treatable condition that should be considered in the differential diagnosis.
Anemia: Anemia can lead to symptoms such as dizziness, fatigue, and shortness of breath, which might be exacerbated by standing. It's a condition that could mimic some aspects of POTS.
Hypothyroidism: Although less directly related to orthostatic symptoms, hypothyroidism can cause fatigue, dizziness, and other nonspecific symptoms that might be considered in the differential diagnosis of POTS.

Orthostatic Hypotension: This condition involves a significant drop in blood pressure upon standing, which can lead to severe dizziness or fainting. It's crucial to differentiate from POTS, as the management strategies can be quite different.
Cardiac Arrhythmias: Certain arrhythmias can cause symptoms similar to those of POTS, including palpitations and dizziness. Identifying an arrhythmia is critical because some types can be life-threatening if not properly managed.
Adrenal Insufficiency: This is a rare condition where the adrenal glands do not produce enough cortisol and, in some cases, aldosterone. It can cause dizziness, fainting, and other symptoms that might be confused with POTS, and it requires prompt diagnosis and treatment to prevent serious complications.

Mitral Valve Prolapse: Although not typically associated directly with POTS symptoms, mitral valve prolapse can cause palpitations, chest discomfort, and other symptoms that might be considered in a broad differential diagnosis.
Dysautonomia: This is a broader category of disorders that affect the autonomic nervous system, of which POTS is a part. Other forms of dysautonomia could present with similar or overlapping symptoms.
Ehlers-Danlos Syndrome: Some patients with Ehlers-Danlos Syndrome, particularly the hypermobile type, may experience symptoms similar to POTS due to autonomic dysfunction. This diagnosis is important to consider in patients with a history of joint hypermobility and other systemic symptoms.