Are you sure that TurboTax isn't asking if box 2b Taxable amount not determined is marked. For a distribution from an inherited traditional IRA, box 2b Taxable amount not determined should be marked, box 7 should have code 4 and the IRA/SEP/SIMPLE box should be marked. If neither box 2b Taxable amount not determined nor the IRA/SEP/SIMPLE box is not marked, the distribution is not from any type of traditional IRA but is instead a distribution from an employer plan like a 401(k).
If the distribution is from an IRA, the marking (or not) of box 2b Total distribution is irrelevant. It has no bearing on how the IRA distribution is taxed.
