Which of the following is a side effect of lithium as indicated by the ADE mnemonic?

Tremors are a well-documented side effect of lithium therapy, particularly at therapeutic levels. This phenomenon is often related to the effect of lithium on extracellular sodium and the stabilization of cell membranes, which can lead to neuromuscular excitability. Patients taking lithium may notice fine tremors, especially in the hands, which can be exacerbated by factors such as stress, caffeine, or dose changes. The presence of tremors is one of the common aspects clinicians monitor when managing patients on lithium, as it can impact medication adherence and overall quality of life.

In contrast, while other options might be seen in various contexts or medications, they are not typically associated with lithium use. Gastrointestinal bleeding and urinary retention are generally more indicative of other medical issues or drug side effects, while rapid heart rate is not a classic side effect of lithium. Hence, tremors distinctly align with the known adverse effects of lithium therapy, making it the correct answer in this context.