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In Breaking Bad, Brock is poisoned by some substance contained in the lily of the valley, as the detectives admit to Jesse as they release him from custody.
First of all, although Walt did indeed get hold of the ricin, he pretends to find it, or a replica, on Jesse's floor, after he convinces Jesse that Gus poisoned Brock.
At that point, in Jesse's mind, Gus poisoned Brock with lily of the valley but wanted him to believe that Walt poisoned the kid with ricin.
Later on, in the final season, realising Huell lifted the ricin (or replica) from his pocket, he shifts over to Walt's house pouring gasoline all over the carpet. But why does he then believe Walt to have poisoned Brock? Has he forgotten about the tox screen results?