Final answer:
A teratogen is an agent that causes birth defects in a developing embryo or fetus as a result of exposure during pregnancy, with the severity and type of defects depending on various factors including potency, dose, and timing of exposure.
Step-by-step explanation:
A teratogen is a biological, chemical, or physical environmental agent that causes damage to the developing embryo or fetus. The question asks, 'What does a teratogen cause?' The answer is (a) birth defect. Teratogens have the potential to cause permanent deformations in the function or structure of a developing embryo or fetus in utero. The effect of a teratogen on the developing organism can vary based on factors such as the potency of the teratogen as a mutagen, the susceptibility of the fetus to teratogenesis, the dose and duration of teratogen exposure, the time of exposure, and the degree of transfer from maternal to fetal circulation. Examples of teratogens include alcohol, which can lead to fetal alcohol spectrum disorders (FASD) characterized by a range of physical and cognitive defects. Other teratogens, like certain drugs, radiation from X-rays, and nutritional deficiencies during pregnancy (such as lack of folic acid leading to spina bifida), are associated with various birth defects.