Without appropriate therapeutic monitoring, taking both prasugrel and aspirin for post-myocardial infarction care increases the risk of:

Toxic epidermal necrolysis is a severe skin reaction that is not typically associated with the use of prasugrel or aspirin. This condition is more often linked to certain medications, particularly anticonvulsants and antibiotics, rather than antiplatelet therapy. Therefore, it does not relate to the risk of combining these two drugs.

Photosensitivity refers to an increased sensitivity to sunlight, often resulting in skin rashes or burns upon exposure. This side effect is more commonly associated with certain antibiotics and other drug classes, rather than with prasugrel or aspirin. The combination of these antiplatelet agents does not elevate the risk of photosensitivity.

Dehydration is generally caused by fluid loss due to various factors such as gastrointestinal issues, excessive sweating, or inadequate fluid intake. It is not a direct consequence of taking prasugrel and aspirin, nor is it influenced by their antiplatelet effects. Thus, it is irrelevant in the context of the risks associated with these medications.