Which of the following is NOT a criterion for Medicare eligibility?

The option indicating "working full-time" is indeed not a criterion for Medicare eligibility. Medicare primarily focuses on specific conditions that allow individuals to qualify, such as age and health conditions. Those who are 65 years or older are eligible for Medicare, which directly addresses the age criterion.

Additionally, individuals under 65 can qualify if they have a permanent disability, meaning they have received Social Security disability benefits for at least 24 months. Another specific eligibility criterion includes individuals diagnosed with end-stage renal disease (ESRD) who require dialysis or a kidney transplant.

In contrast, working full-time does not affect one's eligibility for Medicare. Whether a person is employed or not, they can qualify if they meet the other established criteria. Thus, this option is correctly identified as not being related to Medicare eligibility.