How

[Xinfu]

p.259
-On the other hand, if so, how account for X?

What do the bold words mean? Usually we have 'how to', but even 'how to' does not work, because a verb is lacking.

[Rusty]

I don't like the sentence at all. 'On the other hand' and 'if so' are a strange combination.

You're right that 'how to account' would be correct.
A sentence fragment ('how to account for X' has no verb) is allowed, unless writing a formal paper.

[Xinfu]

Excellent answer. Thank you.