Which condition is characterized by the presence of a blood clot in the stomach of a pig during post-mortem examination?

The presence of a blood clot in the stomach of a pig during a post-mortem examination is indicative of a gastric ulcer. Gastric ulcers can lead to significant internal bleeding, resulting in blood clots forming within the stomach. These ulcers are often caused by factors such as stress, poor diet, and certain medications, and they can severely affect the health of pigs. When an ulcer becomes severe, it can rupture and lead to hemorrhage, which may be observed as a blood clot upon examination.

In contrast, the other conditions listed—kidney failure, pneumonia, and parasitism—are not typically associated with blood clots in the stomach. Kidney failure primarily affects the urinary system and can manifest in other organs but does not lead to gastric blood clots. Pneumonia is a respiratory condition and would present different signs. Parasitism involves the presence of parasites within the animal but does not directly correlate with the formation of blood clots in the stomach. Thus, the characteristic signs of gastric ulcers align directly with the observation of blood clots in the stomach during a necropsy.