Step 5 considers whether a person can make the vocational adjustment needed to perform other types of work. Specifically, does the person have the ability to make an adjustment to any other type of work, considering that person's physical and mental limitations, their age, their education, and their past work experience? At this point Social Security has the "burden of proof" to show that there are jobs in significant numbers that the person could still do despite their limitations, other than what the person did in the past.
In Mary's case, it may well be that she can't do any kind of work that would require her to be on her feet all day because of her chronic back pain. So Social Security will try to determine if she could still do some kind of sedentary full-time job - maybe as receptionist at an office for example - where she wouldn't have to stand all day. But if her back pain would also interfere with her ability to sit for two hours at a time, then even a sit-down type of job would be ruled out for her. At that point, she would be found to have met the definition of "disability."
One last note: Social Security does recognize that as a person gets older, it gets more difficult to transition to other types of jobs. At age 50 and then again at age 55, even if a person were still able perform certain types of jobs, Social Security will award disability benefits under certain circumstances but only if the person is unable to perform his or her PRW as discussed at Step 4.
For more information see: 20 C.F.R. Section 404.1520(a)(4)(v).