Final answer:
The vastus lateralis is a muscle in the upper leg that is often used as the injection site for immunizations in infants and young children. Its use in children under 7 months of age is due to its larger and less developed nature compared to other leg muscles.
Step-by-step explanation:
The vastus lateralis is a muscle located in the upper leg. It is commonly used as the injection site for immunizations, especially in infants and young children. When administering immunizations to children under 7 months of age, healthcare providers often choose the vastus lateralis muscle because it is larger and less developed than other muscles in the leg, making it a suitable site for injection.