heart's output or output of the heart

I don't think there is a GMAT-term difference , however the possessive noun usage ('s) can only act as antecedent to possessive pronouns(its). If you use 's usage you cannot use something like

heart's output under stress remain essentially unchanged as it is a vital organ. =e> it cannot refer to heart

a) output of the heart under stress remain essentially unchanged
(b) heart's output under stress remain essentially unchanged

I do see a difference .
A suggests the output of "heart under stress" remain essentially unchanged.
B suggests that Output itself is under stress.

Any thoughts ?

I dont see any change in both of these options.
Since they follow such as, it is termed as an example or citation in GMAT terms and there no use of relative pronouns in that clause also.

But shouldn't it be under stress remain[b]s[/b] unchanged... in both the options.

Output is a singular term rite.

i think we are talking about "important CAPABILITIES such as X remain(s) unchanged"

so in the above sentence i think remain is apt... ofcourse please do feel free to correct me if wrong.

See the OG problem-
135. Spanning more than 50 years, Friedrich Müller
began his career in an unpromising apprenticeship
as a Sanskrit scholar and culminated in virtually
every honor that European governments and
learned societies could bestow.
(A) Müller began his career in an unpromising
apprenticeship as
(B) Müller’s career began in an unpromising
apprenticeship as
(C) Müller’s career began with the unpromising
apprenticeship of being
(D) Müller had begun his career with the
unpromising apprenticeship of being
(E) the career of Müller has begun with an
unpromising apprenticeship of
Logical predication; Idiom
What spanned more than 50 years? It was
Müller’s career that spanned 50 years and
culminated in virtually every honor. Th e correct
subject of the sentence must be Müller’s career.
A Müller’s career, not Müller, should be the
subject of the sentence.
B Correct. Using Müller’s career as the subject
of the sentence solves the modification
problem with spanning … and provides a
logical subject for culminated.
C Apprenticeship of being is an incorrect idiom;
apprenticeship as is correct.
D Müller’s career, not Müller, should be the
subject of the sentence; past perfect tense is
inappropriate; apprenticeship of being is an
incorrect idiom.
E Müller’s career is preferable to the career of
Müller; present perfect tense is incorrect;
apprenticeship of should be apprenticeship as.

akbism... could you please elaborate on how we could use the sc quoted to relate to the question i had in mind...

thanks!! sorry i missed option E.. i was reading your explanations.. dint look at the question.... yup.. this is something to ponder over.. definately!! kudos to u for pulling this sc out buddy