A 5-year-old boy is found floating face-down in a swimming pool. Rescue personnel initiate resuscitation immediately. At the hospital, he is pronounced dead 2 hours later despite aggressive cardiopulmonary resuscitation. At autopsy, the lungs are heavy, waterlogged, and pale; the stomach is distended with water; and there is foam (frothy fluid) in the mouth and nostrils. The heart chambers show no evidence of blood clotting. Which of the following findings is MOST specific for drowning as the cause of death?