[justsomehnguy]

I'm totally okay with clicking Next button.

But if you really insist, then do what @yreg says, but include big, BIG back button at the top. Bonus points if you can make it look like stacked papers on the desktop:

    ───┬───┬───────────────┐
       │   │               │
     1 │ 2 │ <-            │
       │   │               │
       │   │ A. Lorem ...  │
       │   │               │
       │   │ B. Ipsum ...  │
       │   │               │
       │   │               │
       │   │               │
    ───┴───┴───────────────┘

For the mobile in the portrait mode this is not the best option, but still could work if using vertical stack of cards layout:

     ┌─────────────┐
     │ 1           │
     ├─────────────┤
     │ 2           │
     ├─────────────┤
     │ <-          │
     │             │
     │ A. Lorem ...│
     │             │
     │ B. Ipsum ...│
     │             │
     │             │
     │             │
     │             │
     │             │
     └─────────────┘

NB Next button should be present always, even if it would be meaningless because you would advance on a single-choice question automatically. Thing is, sometimes things break and then someone would need to press the Next button.

Also you can use colour coding for multiple-choice questions by highlighting the Next button if anything selected, confirming for the user what they can advance.

And no, grey on white is the worst possible colour scheme ever.

[ryanhn]

Love this idea, will try my hand at this soon.

[whacked_new]

thanks for running through the thought experiment, this is pretty clever!