When doing medical research with human subjects, which four limitations are unavoidable?

While doing medical research on the human subject, following limitations are unavoidable:

a). Privacy: The privacy of the subject involved in the research should be maintained.

b). the subject must be allowed to the minimal exposure to risks and it must be tested on animals first to avoid deaths.

c). The risk associated with the physiological and physical well being of the subject must be considered.

d). The subject must be well instructed about all the risk and procedure involved in the research.

At the time of performing research with human subjects, there are limitations that can be unavoidable are as follows:  

a. Firstly there will be ethical concerns, that is, there are privacy or ethical concerns to look after.  

b. It is generally not possible to repeat trials on a similar subject.  

c. It can be tough to monitor all the probable variables.  

d. The subjects may report an imprecise medical past.  

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