Which of the following can cause a false-positive fecal occult blood test?

Citrus fruit can indeed lead to a false-positive result in a fecal occult blood test due to the presence of certain compounds such as ascorbic acid (vitamin C). When consumed in significant amounts, ascorbic acid may alter the chemical reactions used in the test, resulting in a misleadingly positive outcome, even in the absence of actual blood in the stool.

While whole grains, leafy green vegetables, and lean meats do contain substances that could potentially affect the test, they are not primarily known for causing false positives to the same extent as citrus fruits. For instance, leafy greens contain Vitamin K, which might affect clotting but are not directly implicated in the test's chemistry, and lean meats are typically associated with potential false-negative results due to the heme in blood. It’s essential for individuals to avoid citrus and specific other foods before undergoing this test to ensure accurate results.