Final answer:
The indication of ferrous sulfate having a therapeutic effect is an Increased tolerance to exercise, as this reflects improved oxygen carrying capacity of the blood due to better hemoglobin levels, which is the treatment goal for iron deficiency anemia.
Step-by-step explanation:
A nurse is collecting data from a client who is taking ferrous sulfate orally. The finding that should indicate to the nurse that the medication is having a therapeutic effect is the client reporting an 'Increased tolerance to exercise'. Iron is a critical component of hemoglobin, which carries oxygen to the body's tissues. Increased tolerance to exercise suggests that the person's hemoglobin levels may have improved, allowing muscles to receive more oxygen and work more efficiently. This improvement is a goal of ferrous sulfate therapy in treating iron deficiency anemia.
Conversely, passage of a soft, formed stool daily might not directly indicate the effectiveness of ferrous sulfate in treating anemia, but rather could reflect normal gastrointestinal function. Decreased number of viral illnesses and improved ability to fall asleep are not typically associated effects of iron supplementation and would not be considered indicators of the therapeutic effectiveness of ferrous sulfate for anemia treatment. While iron plays a role in the immune system, its therapeutic effect in this context is specifically related to correction of iron deficiency and associated symptoms, such as exercise intolerance due to anemia.