Final answer:
The adverse effect of methotrexate for a client with RA is thrombocytopenia, which causes a decrease in platelet count due to the interference with DNA component biosynthesis.
Step-by-step explanation:
A nurse caring for a client taking methotrexate for RA (rheumatoid arthritis) should identify thrombocytopenia as an adverse effect of this medication. Methotrexate works by inhibiting folic acid reductase, essential for converting folic acid to tetrahydrofolic acid.
This conversion is crucial for purine and pyrimidine biosynthesis, which are fundamental components of DNA. By interfering with this process, methotrexate can cause a reduction in platelet production leading to thrombocytopenia, a condition where the blood has an abnormally low number of platelets.