When pluralizing the word cortex, drop the -ex and replace it with -ices.
The grammatical rule the medical assistant would use when pluralizing the word cortex is to drop the -ex and replace it with -ices. This follows the general rule of pluralizing nouns by adding -es when a word ends with s, x, z, sh, or ch. Examples of other words following this rule include boxes, dishes, and churches.
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