From IRS Pub 590-B:
You are considered disabled if you can furnish proof that you can't do any substantial gainful activity because of your physical or mental condition. A physician must determine that your condition can be expected to result in death or to be of long, continued, and indefinite duration.
The IRS generally doesn't do much to ensure that RMD obligations are met, particularly by beneficiaries. If you meet the IRS definition of disabled (substantiated by a doctor's statement), you are an Eligible Designated Beneficiary and your RMDs are based on your life-expectancy from the Single Life Expectancy table based on your age in 2024, reduced by 1 each subsequent year. For age 57, the life expectancy factor is 29.8, so your 2024 RMD is the 12/31/2023 balance divided by 29.8. With that life-expectancy factor, you would not have to fully drain the inherited IRA until 2053. (Successor beneficiaries continue your RMD schedule and would have to drain the inherited IRA by the end of the 10th year following the year of your death if that would mean draining it before 2053).
You would need to provide the doctor's statement to the IRS only if they ask for it or somehow challenge you failing to drain the inherited IRA by the end of the 10th year.
Of course you can always take out more than the RMD if it makes sense. If you expect your marginal tax rate to remain the same in the future, you might want to take distributions that will result in topping out your current tax bracket since you could then invest the money outside the inherited IRA in capital investments where subsequent gains could be taxed at long-term capital gains rates rather than as ordinary income.
