The nurse observes that a client is experiencing melena. Which serum laboratory test should the nurse monitor in response to this finding?

A. Hematocrit. Melena (black, tarry stools) indicates gastrointestinal bleeding, and hematocrit levels can help assess the extent of blood loss and anemia. B. Glucose. Glucose levels are not directly related to melena or gastrointestinal bleeding. C. Blood urea nitrogen (BUN). BUN levels may increase with gastrointestinal bleeding due to the breakdown of blood proteins, but this is secondary to hematocrit monitoring. D. White blood cell count (WBC). WBC count is more related to infection or inflammation rather than direct assessment of gastrointestinal bleeding.