AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020

Parenthetical Plurals
Plurals

When s or es is added parenthetically to a word to express the possibility of a plural, the verb should be singular. However, in most instances it is preferable to avoid this construction and use the plural noun instead (see 7.5.4, Parenthetical Plurals).

Acceptable: The mechanism(s) of this disease process is still unclear.

Better: The mechanisms of this disease process are still unclear.