Does a blood test invalidate or break the fast, and are there certain verses – such as Ayatul-Kursi – that are not to be recited in prayer?

Taking blood from a fasting person for analysis does not invalidate the fast, but it is better to avoid it if it weakens them and causes them to break their fast or become too weak for worship. Reciting any verse from the Quran in prayer is permissible, including Ayatul-Kursi, for it is the greatest verse in the Book of Allah.