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

A. Blood urea nitrogen (BUN) levels may be elevated in cases of gastrointestinal bleeding due to the breakdown of blood in the intestines, but it is not the primary test to monitor for blood loss. B. Glucose levels are important to monitor for clients with diabetes or metabolic disorders, but they are not directly related to melena, which is a sign of gastrointestinal bleeding. C. Hematocrit is a critical lab test to monitor in response to melena, as it measures the proportion of red blood cells in the blood. A decrease in hematocrit indicates blood loss, which is a common consequence of gastrointestinal bleeding. D. White blood cell count (WBC) is used to monitor infection or inflammation, but it is not directly related to the immediate concern of blood loss associated with melena.