Answering Oxbridge medical questions: water regulation by the kidneys

Oxbridge medical questions look for 'how you think' rather than knowledge above A-level standard. For a particular topic, they may start on A-level material to warm you up, and then ask follow-up questions to stretch your logical thinking. Here's an example.
How is water in the body regulated by the kidneys?
This is a question asking about A level Biology material. A good approach is to keep your answer structured, for example, based on the course of the nephron (the tubes where reabsorption of water and ions takes place in the kidney). 1) Glomerulus: water and ions move into the nephron from the blood vessels along osmotic and pressure gradients 2) Proximal convoluted tubule: glucose and water are reabsorbed from the nephron 3) Loop of Henle: reabsorption of sodium happens along the ascending limb, so water is reabsorbed from nephron down its osmotic gradient along the descending limb. This is counter-current exchange 4) Descending convoluted tubule: further water reabsorption 5) Collecting ducts: further water reabsorption; this is controlled by ADH which acts to increase water reabsorption by stimulating the insertion of collecting ducts into the nephron cell membrane

ADH can sometimes malfunction in disease. The body's brain can produce insufficient amounts, or the kidneys can fail to respond to the ADH. We distinguish the two using a water deprivation test - here, patient doesn't drink water over the course of the day, and we measure the concentration of their blood and urine. Halfway through the test we give them a synthetic version of ADH. How could this help us distinguish whether there is a failure of ADH production by the brain or failure of response by the kidney?
Let’s start of by thinking about the effect of depriving someone of water. A lower level of water in the body would result in both their blood and the urine produced becoming increasingly concentrated. We can then tackle this by thinking about the function of ADH. We've answered the 'A-level' question, and understand that ADH stimulates water reabsorption from the nephron. So without the action of ADH, the body won’t be able to act to conserve water, so the blood will continue to become more and more concentrated. We know from the question that this happens either when: 1) The brain produces insufficient ADH 2) Or the kidneys aren’t responding to ADH even if it’s being produced normally. The role of the synthetic version of ADH is where we can distinguish 1 from 2. If the situation is 1, the kidney cells can still respond to ADH, which stimulates water reabsorption from the nephron, so the patient’s blood will start to become less concentrated when the synthetic ADH is given. But if the situation is 2, adding synthetic ADH would make no difference, because the kidneys still won’t be responsive to it, so the patient’s blood will continue to get increasingly concentrated.