Yes, you are correct. In this sentence, the word "lie" is the proper word to fill the blank. The verb "lie" means to recline or be in a horizontal position without an object. In this context, it is used in the present tense to indicate that the patient must remain in a flat position without moving during the X-ray. The word "lay" is the past tense form of the verb "lie" and requires an object, such as "The patient must lay the book on the table." Since there is no object mentioned in the sentence, "lie" is the appropriate word.
