A 77-year-old woman is brought to the emergency room because of symptoms of nonspecific abdominal discomfort. She has no anorexia, fever, chills, or weight loss. On physical examination, the vital signs are normal, the abdomen is soft and nontender with no masses or organomegaly palpated. Abdominal x-rays show lots of stool in the colon, but no free air or air- fluid levels. Laboratory investigations reveal an amylase of 150 U/L (25-125 U/L), and the rest of her biochemistry and complete blood count are normal. Which of the following conditions can cause a false-positive elevation in the serum amylase?