Plural noun + singular or plural noun?

You have placed the possessive to indicate a group (patients')

Use the 3rd plural person verb to match.

For singular possessive the apostrophe is before the s:

The patient's diet was investigated.

For plural possessive the apostrophe is after the s:

The patients' diets were investigated.

The diet was investigated, not the patients, so the verb should agree with the subject, right?

Rewriting below, which one is correct? Should I use "hearts" even though nobody has more than one heart?

The heart of the patients was screened for LVH.
The hearts of the patients were screened for LVH.

Thanks again.

No. Plural patients, plural hearts, plural verb.


Yes.

But patients can share the same diet:

The diet of the patient was rich in antioxidants
The diet of the patients was rich in antioxidants