Final answer:
One rare side effect of using sulfonamides is a deficiency in folic acid, which can lead to various symptoms such as anemia and gastrointestinal disturbances.
Step-by-step explanation:
One rare side effect that may result from using sulfonamides is a deficiency in folic acid. Sulfonamide drugs competitively inhibit the synthesis of folic acid, leading to a deficiency that can affect DNA synthesis and cell division. Some signs of deficiency include magaloblastic anemia, glossitis, gastrointestinal tract disturbances, and leucopenia.