Final answer:
Antidiuretic hormone (ADH) must be present for water to osmotically follow sodium in the collecting duct. ADH makes the collecting ducts in the kidneys permeable to water, allowing water to be reabsorbed back into the blood through osmosis.
Step-by-step explanation:
The hormone that must be present for water to osmotically follow sodium in the collecting duct is antidiuretic hormone (ADH), also known as vasopressin. ADH is produced in the hypothalamus and acts on the kidneys. It makes the collecting ducts in the kidneys permeable to water, allowing water molecules to be reabsorbed from the urine back into the blood through osmosis into capillaries.