Sierra's chemistry professor, Dr. Eskandari, suspects Sierra might have a learning disability. Her performance in his course is inconsistent. She seems to perform poorly on tests even though she always attends and participates in class, visits him during office hours for extra help, and claims to study very hard for quizzes and exams. Dr. Eskandari has noticed that Sierra has trouble paying attention in class and that her homework is frequently messy and appears to be carelessly completed. What learning disability does it sound like Sierra might have?