What happens if a person is unable to secrete ADH?

The question asks what is most likely to happen if a person is unable to secrete ADH (Antidiuretic Hormone).

• ADH's primary function is to increase the permeability of the renal tubules (specifically the collecting ducts and distal convoluted tubules) to water. This allows more water to be reabsorbed back into the blood, concentrating the urine and reducing its volume.
• If a person cannot secrete ADH, the renal tubules will remain less permeable to water. This means less water will be reabsorbed from the filtrate back into the bloodstream.
• Consequently, a larger amount of water will remain in the filtrate, leading to the production of large volumes of dilute urine.

Let's evaluate the options:

• A: "the renal tubules will become more permeable to water." - This is incorrect. Lack of ADH makes them less permeable.
• B: "the blood will have a higher volume of water." - This is incorrect. Less water reabsorption means more water is lost from the body, leading to a lower blood volume.
• C: "large volumes of urine will be produced." - This is correct. Without ADH, less water is reabsorbed, resulting in increased urine volume.
• D: "more water is reabsorbed by the renal tubules." - This is incorrect. Less water is reabsorbed without ADH.

The correct option is C.

C. $\boxed{\text{large volumes of urine will be produced.}}$