Which plant causes primary photosensitization?

• A. St. Johnswort
• B. Redroot Pigweed
• C. Beets
• D. Corn

Answer: (A)

Primary photosensitization occurs when a plant contains a photodynamic compound that is active in tissues without requiring liver processing. After ingestion, this compound accumulates in the skin, and when the animal is exposed to sunlight, it is activated by UV light to cause tissue damage.

St. John’s wort is a classic example because it contains hypericin, a photodynamic pigment. Once eaten, hypericin can accumulate in the skin, and sunlight triggers a reactive, damaging response in exposed areas such as the face, ears, and udder. That direct, light-activated damage sets it apart as a primary photosensitizer, independent of liver function.

The other plants listed are not recognized as typical primary phototoxins in this context, so they don’t fit the mechanism as clearly.