Hi! I'm not sure which question you are referring to -- when I look at ICE3 Q60, it says "In what way is the blood in the pulmonary artery most likely to differ from an ideal fluid?". This doesn't sound like the same question you are asking.
In general, Poiseuille's law is used when the system is not ideal and friction must be taken into account. Does that answer your question? If not, please email the instructor with a screenshot of the problem you are referring with your question and either I or him will get back to you.