Hypothesis

Hypothesis is a repeatable quest in Fallout 4.

Quick walkthrough
Clayton Holdren asks the Sole Survivor to procure a tissue sample from a particularly strong super mutant for his experiments. Head to the supplied coordinates, collect the sample from the marked super mutant and return for your reward.

Detailed walkthrough
This quest can be received from Clayton Holdren at various points throughout the Institute questline. If he doesn't give it to you on his own accord as soon as you talk to him, ask if he needs anything done.

Once active, the quest will direct you to a randomly chosen super mutant stronghold like Faneuil Hall or the Scrap Palace, where you'll be required to kill the resident super mutant leader. The exact enemy type is dependent on player level. Said leader will drop the requested Flesh Sample upon death.

With the sample in your possession, return to Holdren for a reward in experience points. Note that in order to get any material reward from him, an easy charisma check needs to be passed. If done successfully, Holdren will usually part with some medical supplies, e.g. a couple of Stimpaks.

Bugs
Attempting to persuade for a reward after doing this quest more than once will give dialogue for the failed persuasion attempt even if the charisma check is passed, therefore Clayton will not reward the 4 Stimpacks.