Final answer:
The symptoms of weakness, lethargy, and confusion following a craniotomy are indicative of hyponatremia, an electrolyte imbalance related to low sodium levels in the blood.
Step-by-step explanation:
A patient becoming weak, lethargic, and confused following a craniotomy could be experiencing an electrolyte imbalance. Of the options listed, hyponatremia, which is an abnormally low sodium level in the blood, best fits the symptoms described. Hyponatremia can lead to confusion, lethargy, and weakness, among other symptoms. In contrast, hyperkalemia affects the muscular and cardiac functions more directly, and while it can cause mental confusion, its primary indicators are often related to muscle and heart activity.