Why is TT, despite having more than $50K (after deducting 10% from AGI) in itemized deductions (mostly medical), referring me to take the Standard Deduction of $15,500?

It could be you are filing separately and indicated your spouse will be taking the standard deduction or you are not understanding the summary screen correctly ... do not look at the deductions & credits summary column since you cannot count the credits for Sch A deductions.   

Maybe if you look at the actual Sch A you will see why ... 

 If it's giving you the Standard Deduction and  not showing you Schedule A you can check the actual amount of itemized deductions by using by going to:

Tax Tools on left

Tools - Topic Search (top left box)

Type in  itemized deductions, choosing.  It should highlight that in the list, click on GO

Then Click on "Change my deduction". That will display the actual amount of itemized deductions vs. the standard deduction.    (Be sure to uncheck "Change my deduction" after checking it so you do not lock in the wrong deduction.

How to change between the Standard Deduction and Itemized Deductions

https://ttlc.intuit.com/community/accessing/help/how-do-i-change-from-the-standard-deduction-to-item...