Final answer:
The full faith and credit clause in Article IV, Section 1 of the U.S. Constitution ensures that a Pennsylvania marriage license is valid in Maryland. This clause requires states to recognize the public acts and legal decisions of other states, including marriage licenses.
Step-by-step explanation:
The clause that allows your Pennsylvania marriage license to be valid in Maryland is Article IV, Section 1 of the United States Constitution, commonly referred to as the full faith and credit clause. This clause mandates that each state must recognize the public acts, records, and judicial proceedings of every other state. Marriage licenses are included in this category, and therefore, a license issued in Pennsylvania is recognized as valid in Maryland. It's important to note that despite the full faith and credit clause, legal complexities such as the Defense of Marriage Act (DOMA) had impacted the recognition of certain marriages across state lines. However, since the Supreme Court rulings against the provisions of DOMA, marriage licenses, including those for same-sex marriages, must be recognized in all states.