correct answer is (c).
Although it is
true that this approach will more precisely define the cases
under study, the final result may or may not fulfill the
study aims. On the plus side, the distribution of cases
available for analysis at the end of the study will probably
include mostly clinically relevant cases. This perhaps will
allow a more clear description of the type of fatigue associated
with this particular therapy. On the minus side however,
the approach may exclude patients with milder fatigue, which
could lead to a description that misses part of the phenomenon,
or yield a skewed distribution for statistical analysis.
This could complicate the interpretation of the data or
lead to questions about the generalizability of the results.