Delusional disorder

A 30-year-old woman is brought to the physician by her parent. For the last three months, the patient has been telling her parent that a famous singer is in love with her and is trying to reach out to her through the lyrics of his songs. The patient tells the physician that she knows he is in love with her and is trying to divorce his wife before officially getting in touch with her – therefore, to avoid any legal consequences, he has to contact her through his songs. Although her family has tried to convince her otherwise, the patient has been extremely stubborn about this belief, and she says that he will marry her as soon as he divorces his wife. The patient is working as a secretary for a law firm and is doing well in her job. She enjoys spending time with her friends and cooking during family gatherings. She does not use alcohol or recreational drugs. A urine drug test is negative. Which of the following is the most likely diagnosis?