Anurse is caring for a client who is three hours post op from open abdominal surgery. during routine assessment, the nurse notes the previously stable client now appears anxious, apprehensive, and has a blood pressure of 90/56. what does the nurse consider is the most likely cause of the client's change in condition?

Anxious and apprehensive are signs that the patient doesn't have enough blood to his brain. The cause probably is the low blood pressure. 
Open abdominal surgery is a major surgery so it should have a high risk of bleeding. Bleeding in abdominal space is dangerous because it was not visible and space is massive. The source could be a rupture of a blood vessel or broken stitch of the surgical wound. 
Then, the most likely cause for the sudden blood pressure drop in this patient would be massive bleeding.