In Act 5 scene 1, what conclusion does the doctor reach regarding Lady Macbeth?
In order to understand the doctor's conclusion regarding Lady Macbeth's behavior, we first need to understand the Elizabethan physician (Shakespeare lived and wrote in the Elizabethan era, so his writing reflects and is informed by life in that era). First, only the very wealthy could afford an actual physician (rather than a surgeon, a barber, an apothecary, or the Church, all of which were considered inferior in knowledge and skill to a physician). A physician would have had a college degree and then been further educated in medicine at the College of Physicians. Because Lady Macbeth was royalty, she would have been seen by an actual physician. However, and ironically, even as people became more educated during this Renaissance period, belief in witches, witchcraft, and the supernatural was renewed, and this belief extended even to those in the sciences, including physicians. Most of what physicians of this period believed was based on Aristotelean and Hippocratic teachings on the "humors," and the "cures" were often either bloodletting or rituals based on astrology. Needless to say, there was very little actual medical care happening!
Now that we have a very basic understanding of the Elizabethan physician, we should look to Lady Macbeth's behaviors:
In the opening scene of Act 5, the doctor is speaking to a "waiting gentlewoman." In today's English, this equates to a kind of personal assistant to a noblewoman. Her job would have been to help her lady dress and undress; to care for her Lady's clothing; to help her Lady bathe, do her hair, and attend to other aspects of physical appearance; and to take care of her Lady's bedchamber and chamber pot. A Lady's gentlewoman and a Lord's gentleman were often the highest-paid and most trusted of servants. In this role, the waiting gentlewoman would have been privy to most of her Lady's behaviors...and secrets. Understanding this helps us to understand why the doctor questions her so closely and why the gentlewoman knows so much of what is happening with Lady Macbeth.
What is happening with Lady Macbeth? Her gentlewoman tells us that she has "seen her rise from her bed, throw her nightgown upon her/unlock her closet, take forth paper, fold it, write upon't/read it, afterwards seal it, and again return to bed; yet all this/while in a most fast sleep" (lines 4-7). The doctor responds, "A great perturbation in nature, to receive at once the/benefit of sleep and so the effects of watching!" (lines 8-9). In other words, it is unnatural to perform waking deeds while asleep. He asks the gentlewoman if she has heard Lady Macbeth speak while in this state, and she replies that she has but that she will not tell him. At this point, Lady Macbeth enters with a candle and performs the act of washing her hands, which her gentlewoman tells the doctor she will sometimes do for 15 minutes, while speaking of the sight and smell of much blood. The doctor exclaims, "Go to, go to! You have known what you should not" (line 40). Most interpret this to mean that the doctor knows at this point that Lady Macbeth has committed murder, and he understands Lady Macbeth's "heart is sorely charged" (line 45). After Lady Macbeth heads back to bed, the doctor muses, "Foul whisperings are abroad. Unnatural deeds/Do breed unnatural troubles" (lines 62-63). Here, the phrase "Foul whisperings are abroad" and the word "unnatural" both indicate the doctor is not only aware of but believes in the supernatural and helps us to understand this means something far more than "not normal." He concludes, therefore--and his conclusions are informed by the religious, cultural, and societal frameworks of his day as well as by what he has just witnessed--that "[t]his disease is beyond my practice" (line 50) and that Lady Macbeth "[m]ore needs . . . the divine than the physician" (line 65). In other words, the doctor sees Lady Macbeth's behavior as something driven by a sickness of the heart and soul rather than one of the body, and she needs a priest more than she needs a physician.
The doctor observes Lady Macbeth's behavior in her sleepwalking incident. He goes to Macbeth and tells him that he is not able to cure her, that she needs God's help to do that. The doctor has realized that it is her conscience that has driven her to this illness, and he would not be able to help her with that.
The doctor will also realize what Lady Macbeth is feeling so guilty about. Her sleepwalking episodes will confirm what the kingdom has suspected - she and her husband are murderers.