The Privileges and/or Immunities Clause

There are a lot of clauses to know in Constitutional Law but among the ones you should know best is the Privileges and Immunities Clause.

Most importantly for MBE purposes is the clause falling under Article IV, also called the Interstate Privileges and Immunities Clause. The clause prohibits discrimination by a state against nonresidents of that state. "Nonresidents" here does not include corporations or aliens, and this is one way that this clause differs from the Equal Protection Clause, the Due Process Clause, and the Dormant Commerce Clause. 

The Interstate Privileges and Immunities Clause prohibits discrimination by a state against a nonresident only if the discrimination concerns fundamental rights. In this context "fundamental rights" includes commercial activities and civil liberties. 

And even if you see in a question the type of discrimination described above, you should consider whether the state had a substantial justification for the different treatment. The state will want to show that the nonresidents caused or are a significant part of the problem that the state is trying to solve and that there are no less restrictive means other than unequal treatment for solving that problem. That might provide the substantial justification that the state will need in order to defend. 

Most of the questions dealing with privileges/immunities will deal with the Interstate clause under Article IV.  But there is also another clause quite similar and it would be easy to muddle them up. There's a Privileges or Immunities Clause (notice the "or" rather than the "and") under the 14th Amendment. Under this clause, a state may not deny its citizens the privileges and immunities of national citizenship (for example, the right to petition congress for redress, the right to vote, etc.). And once again, corporations are not protected by this clause.