My screen reader (NVDA) will not read the question stem of a Multiple Choice question

Hi Jackie! Tabbing will take you to interactive elements on the slide (and then, as you noticed, you can go between the interactive elements using the up/down keys), but I suspect your question is simply a text box! When you're testing with/using NVDA and your Storyline 360 course, you should be able to just use the "down" arrow to have the screen reader start reading the text-based content on the slide. 

Learners use their screen reader navigation keys (e.g., Down and Up arrows) to move through all text and interactive objects on the slide, listening to each item along the way.

Using Tab and Shift+Tab will jump from one interactive object to another, skipping text and images, which I think is what you're experiencing. More on the accessible slide content is available here!