Answer :
When a pathogen is swallowed, it is most likely to be destroyed by gastric juices.
Gastric juices are secreted by the lining of the stomach and are composed of hydrochloric acid and digestive enzymes. These components create a highly acidic environment with a pH ranging from 1 to 3. Such extreme acidity is very effective in denaturing proteins and destroying many types of pathogens, such as bacteria and viruses, that enter through the digestive system.
Let's look at why the other options are less likely to destroy swallowed pathogens:
Pancreatic juices: These contain digestive enzymes that help in breaking down carbohydrates, proteins, and fats but are secreted into the small intestine, where pathogens have often already been neutralized by gastric juices.
Complement proteins: These are part of the immune system that help to enhance the ability of antibodies and phagocytic cells to clear pathogens. They usually operate in the blood and tissues, rather than the gastrointestinal tract.
Peyer's patches: These are small masses of lymphatic tissue found in the small intestine, playing a role in immune surveillance, but they do not directly destroy pathogens like gastric juices do.
Thus, the acidic environment created by gastric juices is a primary defense mechanism against pathogens ingested with food or drink.