A 45-year-old man is found dead in a bathtub at home. The body is retrieved after approximately 8 hours of submersion in fresh water. On external examination, the pathologist notes a pale, wrinkled appearance of the skin on the hands and feet, frothy fluid in the mouth and nostrils, and a distended abdomen. The lungs are heavy, waterlogged, and edematous. Histologically, diatoms are found in the lungs and bone marrow. Which of the following findings is most specific for confirming that drowning occurred before death, rather than postmortem immersion?