Final answer:
For an infant with a rash potentially due to cow's milk based formula, the most appropriate step is to switch to a protein hydrolysate based formula which has proteins broken down into smaller parts to reduce the likelihood of an allergic reaction.
Step-by-step explanation:
The most appropriate next step for a 2-month-old infant who develops a full body rash after being exclusively fed with cow's milk based formula would likely be to switch to a protein hydrolysate based formula. Cow's milk proteins can be difficult for an infant to digest and can lead to allergic reactions, which might present as a rash. A protein hydrolysate formula contains proteins that are broken down into smaller peptides, making them easier to digest and less likely to trigger an allergic reaction.
In contrast, switching to a soy protein-based formula might not be the best initial move, as some infants who are allergic to cow's milk protein could also react to soy protein. A formula with high MCT (medium-chain triglycerides) oil is often used for malabsorption issues, but does not address potential allergic responses to protein. Lastly, a lactose-free cow's milk-based formula would not be suitable if the issue is a protein allergy rather than lactose intolerance.