For a 79-year-old man with lung cancer and a sodium level of 122 mmol/L, what is the most appropriate management?

In the case of a 79-year-old man with lung cancer presenting with a sodium level of 122 mmol/L, the most appropriate management is fluid restriction. This is a common approach for managing hyponatremia, particularly when it's caused by conditions such as the syndrome of inappropriate antidiuretic hormone secretion (SIADH), which is often seen in patients with malignancies, including lung cancer.

Fluid restriction helps to decrease the total body water, thereby allowing serum sodium levels to rise. When there is excess fluid in relation to sodium, limiting fluid intake can help restore the balance between body sodium and water, leading to an increase in serum sodium concentration.

Other options focus on different mechanisms of action, such as using medications that either block the effects of antidiuretic hormone or promote water excretion. However, in many cases of mild to moderate hyponatremia, particularly due to fluid overload, fluid restriction remains the most straightforward and effective initial management strategy.