If you touched a poison oak plant and developed itchy blisters, what substance caused that reaction?

The substance responsible for the reaction of itchy blisters after contact with a poison oak plant is urushiol. Urushiol is an oily organic allergen found in the leaves, stems, and roots of poison ivy, poison oak, and poison sumac. When the skin comes into contact with urushiol, it can trigger an allergic reaction in many individuals, leading to symptoms such as redness, itching, swelling, and the formation of blisters. This reaction is classified as contact dermatitis and results from the immune system's response to urushiol.

Understanding the other substances listed, oxalic acid is primarily found in certain plants, including some edible ones, and has different properties and effects. Steroidal alkaloids are a class of compounds that do not relate to poison oak. Acetylcholine is a neurotransmitter involved in signal transmission in the nervous system, which does not play a role in the allergic reaction to poison oak. Hence, urushiol is the correct choice, as it directly correlates to the symptoms experienced after exposure to poison oak.