When administering a medication intramuscularly to a child under 5 years of age, the best site to use is the:

The vastus lateralis muscle is considered the best site for administering intramuscular injections in children under 5 years of age due to several important factors. This muscle is located on the outer aspect of the thigh and is well-developed in infants and young children. Using the vastus lateralis minimizes the risk of harming nerves and blood vessels, which are more prominent in other areas, such as the dorsogluteal and deltoid regions.

The dorsogluteal muscle is less advisable for young children because the muscle is not as well developed in infants, which increases the risk of hitting the sciatic nerve or major blood vessels. The deltoid muscle, while it can be used for older children and adults, may not have enough muscle mass to safely administer medication in younger patients. The rectus femoris muscle is another option but is generally not preferred because it is more difficult to access and there are more concerns about potential complications, such as injury to nearby structures.

In summary, the vastus lateralis muscle is not only easily accessible but also safer for intramuscular injections in young children, making it the preferred site for this age group.