A market hog weighing 150 lbs suddenly dies appearing gaunt and extremely pale, with a big blood clot found in its stomach during examination. What is the probable cause of death?

The description of the market hog, including its sudden death, gaunt appearance, extreme pallor, and the presence of a significant blood clot in the stomach, suggests a case of gastric ulceration. Gastric ulcers can lead to severe internal bleeding, which would account for the pale appearance of the animal due to blood loss and anemia. The rapid decline in condition leading to sudden death aligns with acute gastric ulcers, which can progress quickly and lead to significant health issues if not promptly treated. The blood clot found in the stomach supports this diagnosis, as it indicates that there was a rupture or severe erosion of blood vessels in the gastric mucosa, typical of severe ulceration.

While heart failure, pneumonia, and leptospirosis could all lead to health issues in swine, the specific symptoms and findings described in this scenario point to gastric ulcers as the most likely cause of death in this particular case.