Final answer:
The therapeutic effect of ferrous sulfate is indicated by an increased tolerance to exercise, demonstrating improved oxygen transport and cellular respiration due to better iron levels in the blood.
Step-by-step explanation:
The client who is taking ferrous sulfate orally and reports an increased tolerance to exercise indicates that the medication is having a therapeutic effect. Ferrous sulfate is prescribed to treat iron deficiency anemia, a condition in which the body lacks sufficient iron to produce adequate levels of hemoglobin, the substance in red blood cells that carries oxygen to the body's tissues. An increase in exercise tolerance suggests that the hemoglobin levels are likely improving, allowing more efficient oxygen transport and better support for muscular activity. Iron's primary functions include oxygen transport and involvement in cellular respiration, so an improved ability to perform physical activity is a clear sign of the effectiveness of the iron supplementation.