What is the ruling on fasting Ramadan and performing the Sunnah prayers while sitting for a patient with "POTS" whose symptoms, such as severe fatigue, dizziness, rapid heartbeat, and dehydration, worsen with fasting?

It is permissible for a sick person whose illness is expected to be temporary and who is harmed by fasting to break their fast. They must make up for the missed days after recovery, based on the Almighty's saying: (And whoever is ill or on a journey - then an equal number of other days).

However, if the illness is chronic and not expected to heal, then it is permissible to break the fast while feeding a poor person for each day, based on the Almighty's saying: (And for those who can [fast with difficulty], a ransom of feeding a poor person). The ability to fast is determined by experience or by the report of a trustworthy Muslim doctor.

As for Tarawih prayer, it is permissible to perform it while sitting. If one prays sitting while able to stand, they will receive half the reward of one who prays standing. If they are unable to stand, they will receive the full reward.