Final answer:
The vastus lateralis muscle is the common site for intramuscular injections in infants, as it is large and free from major nerves and blood vessels.
Step-by-step explanation:
The muscle commonly used for intramuscular injections in an infant is the vastus lateralis muscle. The vastus lateralis is one of the quadricep muscles located on the lateral aspect of the thigh and is the preferred site for injections due to its size and the lack of large nerves and blood vessels in the area. This makes it safe for administering vaccines and other medications to infants who have small muscle mass.