Final answer:
Lauren's weight gain likely resulted from increased reabsorption of water by the collecting duct in the kidneys, possibly influenced by aldosterone's role in sodium reabsorption, leading to water retention.
Step-by-step explanation:
The student's question asks which of the following directly caused Lauren's weight gain. Based on the information provided, the answer to this question appears to be influenced by the mechanisms of water and sodium balance in the kidneys. The kidneys regulate water balance by reabsorbing water in different parts of the nephron, including the collecting duct. In addition, sodium levels play a crucial role in water reabsorption due to osmosis.
Hence, an increased reabsorption of water by the collecting duct would result in less water being excreted, potentially leading to weight gain due to water retention. Additionally, the hormone aldosterone prompts the kidneys to reabsorb sodium, which in turn increases water reabsorption due to the osmotic gradient created by the reabsorbed sodium. This physiological process indicates that the mechanism described in both options regarding increased reabsorption of water by the collecting duct potentially linked to aldosterone activity could be the direct cause of Lauren's weight gain.