When a base word has a prefix, do we use the rules for a and an on the prefix, or on the base word?

For instance, with the phrase "lexico-orthographical pedant", would we say "an lexico-orthographical pedant" or "a lexico-orthographical pedant?"

My suspicion and inclination is to use the latter, since "lexico" begins with a consonant sound, but I can see the argument for the former.

Thanks!

-Greg.

When using the indefinite article (a/an) simply use the usual rules based on the word which immediately follows it: 'a' when followed by a consonant sound, 'an' when followed by a vowel sound.

In your example the word 'lexico-orthographical' begins with a consonant sound, therefore it should take the indefinite article 'a'.