Four-year-old MacKenzie became very ill a few days after enjoying a picnic with several members of her extended family. When MacKenzie was being treated in the emergency room of a local hospital, the attending physician asked her parents to recall what picnic foods were eaten by their daughter. They remembered that she had lemonade; potato chips; a grilled, rare hamburger in a bun; sliced tomatoes; some watermelon; and a chocolate cupcake. Which of the following food items was the most likely source of the toxin that made MacKenzie so sick?