Aretha's blood and urine tests at a recent checkup show that her levels of iron are low. she is surprised by the results because she doesn't have any of the symptoms associated with low iron, such as fatigue and headaches. why type of deficiency does this suggest?

This suggests subclinical deficiency .

Subclinical deficiency is a condition in which a nutritional deficiency that is capable of having an effect on an individual’s health is not severe enough to produce typical deficiency signs and symptoms. Subclinical deficiency is discovered only by biochemical changes and it can change to clinical deficiency if it is not treated.

the answer is:

carbon dioxide