What are pathogens?

Pathogens are disease-causing agents. They can be bacteria, viruses, fungi, protists, or parasitic worms.
They can infect other organisms and cause various diseases.
Choice A is wrong because phagocytic leukocytes are a type of white blood cell that can engulf and destroy pathogens.
They are part of the immune system and help fight infections.
Choice B is wrong because antibodies are proteins that can bind to specific antigens on the surface of pathogens and mark them for destruction by other immune cells.
They are also part of the immune system and help fight infections.
Choice D is wrong because agents that alter DNA base sequences are not necessarily pathogens.
They can be mutagens, such as chemicals or radiation, that can cause changes in the DNA of cells.
Some mutagens can also be carcinogens, which can cause cancer.

Exocrine glands secrete through ducts or tubes to the body’s exterior.

For example, sweat glands, salivary glands, and liver are exocrine glands.
Choice A is wrong because exocrine glands do not only secrete salts, but also other substances such as enzymes, mucus, and sebum.
Choice B is wrong because exocrine glands do not secrete hormones at all.
Hormones are secreted by endocrine glands, which are ductless glands that release their products directly into the bloodstream.
Choice C is wrong because exocrine glands do not secrete into the bloodstream, but onto an epithelial surface such as the skin or the gastrointestinal tract.

Only endocrine glands secrete into the bloodstream.