Which legal principle requires law enforcement to obtain a warrant before conducting a search?

The Fourth Amendment is the correct answer because it explicitly protects individuals from unreasonable searches and seizures and establishes the requirement for law enforcement to obtain a warrant based on probable cause before conducting a search. This constitutional provision is fundamental in maintaining the balance between individual privacy rights and the government's interest in conducting searches for evidence related to criminal activity.

The Fourth Amendment outlines the need for warrants to be specific in describing the place to be searched and the items to be seized, ensuring that searches are conducted lawfully and respect citizens' rights. This principle is a cornerstone of American law and serves to limit arbitrary governmental intrusion into people’s homes and personal belongings.

The other options touch upon important legal concepts related to evidence and law enforcement, but they do not directly establish the requirement for obtaining a warrant. Probable cause is a standard that must be met for a warrant to be issued, but it does not, by itself, mandate the need for a warrant. The exclusionary rule prevents the use of evidence obtained in violation of the Fourth Amendment, while the fruit of the poisonous tree doctrine extends this principle to exclude evidence derived from illegally obtained information. However, none of these concepts are as directly tied to the requirement of obtaining a warrant as the Fourth Amendment itself.