Final answer:
Mary was pleased about her second correct diagnosis because it offered certainty, suitable treatment options, and emotional relief.
Step-by-step explanation:
Mary was likely pleased about her second correct diagnosis because it provided certainty and reassurance about her health or the condition of the person she was concerned about. Diagnosis in medicine can often be challenging and when a correct diagnosis is made, it offers a pathway to suitable treatment and potentially a good prognosis. Furthermore, a correct diagnosis can relieve the stress and anxiety associated with uncertainty, thus contributing to a patient's or caregiver's emotional wellbeing. Mary's reaction aligns with the sentiment expressed by Dr. Rank in the provided text, where absolute certainty was met with the justification for merriment after achieving a correct diagnosis.