Which root word denotes poison?

The root word that denotes poison is "tox/o." This root originates from the Greek word "toxikon," which means poison or toxic substance. It is commonly used in medical terminology to refer to various types of poisons or toxic reactions within the body, such as in the terms "toxicology" (the study of poisons) and "toxemia" (the presence of toxins in the blood).

The other root words provided have different meanings: "hem/o" refers to blood, "thromb/o" relates to clots, and "splen/o" pertains to the spleen. Each of these roots serves a specific function in describing various components or conditions of the body, but none of them indicate the concept of poison. Therefore, "tox/o" is the only root that directly relates to poison.