Josh works for a federal governmental agency that requires drug testing as a condition of employment. he wants to challenge the constitutionality of the testing in court. for his case, josh will attempt to rely on

Fourth amendment

Explanation:

The Fourth amendment of the US constitution provides protection to  people in their persons, papers, houses and effects, against unlawful searches and seizures, shall not lead to violation and no issue of Warrants. This means that the police does not have the permission to search an individual or a person's house without a probable cause.

Challenges to the policies of drug testing  at workplace on the basis that  they leads to the violation of the employees' privacy have been abortive.

Thus in case of the drug testing as an employment condition, Josh will attempt to Fourth amendment as it does not violate it.