Following delivery, the nurse's assessment reveals a soft, boggy uterus located above the level of the umbilicus. The appropriate intervention is:

Injecting the medication into the vastus lateralis. This is because the vastus lateralis is a large muscle in the thigh that is suitable for intramuscular injections in newborns³. The vitamin K injection helps prevent vitamin K deficiency bleeding, which is a rare but serious condition that can cause bleeding in the brain or other organs¹. The American Academy of Pediatrics recommends that all newborns receive a single intramuscular dose of 0.5 to 1 mg of vitamin K within one hour of birth².

Choice A is wrong because a 21 gauge needle is too large for a newborn's muscle. A 25 or 27 gauge needle is more appropriate.

Choice B is wrong because injecting at a 45-degree angle may not reach the muscle tissue. A 90-degree angle is more appropriate.

Choice C is wrong because injecting 1cc of medication is too much for a newborn's muscle. The recommended dose of vitamin K is 0.5 to 1 mg, which is equivalent to 0.05 to 0.1 mL.

The Moro reflex was elicited. This is because the Moro reflex is a normal newborn reflex that occurs when the baby is startled by a loud noise or a sudden movement. The baby responds by extending the arms and legs, opening the hands, and then bringing the arms and legs back to the chest.

The Moro reflex is present at birth and disappears by 3 to 6 months of age.

Choice B is wrong because this is not abnormal for a full-term infant. The Moro reflex is a sign of a healthy nervous system and brain development.

Choice C is wrong because there is no evidence of an abnormality in the musculoskeletal system. The Moro reflex does not indicate any problems with the bones or muscles of the baby.

Choice D is wrong because the full-term infant should react to sudden movement. The Moro reflex is a protective response that helps the baby cling to the mother in case of danger.