TOPIC: has / have

Which of the following sentences is correct:
A) “The medical examination does not HAS any side effect.” or
B) “The medical examination does not HAVE any side effect.”

Only B is correct because we indicate that the subject is singular in only ONE part of the verb. If there is an auxiliary verb, then the auxiliary verb must be changed to indicate a singular number; if there is no auxiliary, then the main verb is changed (from the base verb, the infinitive without “to”). So, for a positive statement with a singular subject, we add -s or -es to the main verb or, if the verb is irregular (as in the case of “have / has”), we use the singular form, e.g.,
“The examination HAS several side effects.” (positive)
“The examination DOES NOT HAVE any side effects.” (negative)
We can never combine a singular auxiliary form with a singular main verb, e.g.,
**(incorrect) “The examination DOES NOT HAS any side effects.”