Which of the following conditions causes a decrease in serum sodium levels?

A decrease in serum sodium levels can occur due to the syndrome of inappropriate antidiuretic hormone secretion (SIADH). This condition is characterized by excessive release of antidiuretic hormone (ADH), which leads to increased reabsorption of water in the kidneys. As more water is retained in the body, it dilutes the sodium concentration in the serum, resulting in hyponatremia (low sodium levels). The excess water retention alters the electrolyte balance, causing a significant drop in serum sodium despite normal or even elevated total body sodium content.

Other conditions listed are associated with increased sodium levels. For example, hyperaldosteronism typically leads to sodium retention and can potentially cause hypertension, while congenital adrenal hyperplasia often alters hormone levels but does not primarily decrease serum sodium. Cushing's syndrome is associated with excess cortisol, which can lead to sodium retention. Thus, the primary mechanism of SIADH causing dilutional hyponatremia makes it the condition that results in a decrease in serum sodium levels.