Pharmacology

A clinical trial was conducted by orally administering a new analgesic drug to a group of patients suffering from arthritic pain. One hour after administration, the patients were questioned about pain relief, and a sample of blood was collected from each patient to measure plasma drug levels. Pain relief generally correlated well with plasma levels, but three patients whose plasma levels were equal to zero reported good pain relief. Which of the following is the most likely explanation of the analgesic eeffect of the drug in those three patients?

An 85-year-old woman was admitted to the hospital with an infected decubitus ulcer. Cultures revealed Staphylococcus aureus that was sensitive to some β-lactam antibiotics, namely, oxacillin, cephalexin, and imipenem. Which of the following would be thefirst procedure to undertake in order to minimize the risk of an allergic reaction to these antibiotics?

A 32-year-old woman complained to her physician of urticaria. Three days earlier, the woman had started therapy with amoxicillin for infectious tonsillitis. The physician suspected an allergic reaction to the drug. Which of the following was most likely the main determinant of that drug reaction?