**Core Concept**
Lithium is a mood stabilizer commonly used in the treatment of bipolar disorder. Its therapeutic effects are well established, but it also has a range of side effects that can impact patient management. Among these, fine tremors are a well-documented adverse effect of lithium.

**Why the Correct Answer is Right**
Fine tremors are a common side effect of lithium due to its effect on the nervous system. Lithium inhibits the reuptake of catecholamines, such as dopamine and norepinephrine, which can lead to increased neurotransmitter activity. This increased activity can cause tremors, particularly in the hands. The mechanism is thought to involve lithium's blockade of the sodium-potassium ATPase pump, leading to an influx of sodium ions into the neuron. This can cause hyperexcitability of the motor neurons, resulting in fine tremors.

**Why Each Wrong Option is Incorrect**
**Option A:** Polyuria is a side effect of lithium, but it is not the most characteristic or specific side effect. Lithium increases the excretion of sodium and water, leading to increased urine production. However, this is not as closely associated with lithium as fine tremors.
**Option C:** Polydipsia is not a direct side effect of lithium. While patients on lithium may experience polyuria, leading to increased fluid intake, polydipsia is not a characteristic side effect of the medication itself.
**Option D:** Weight gain is not a typical side effect of lithium. In fact, lithium can sometimes cause weight loss due to its effects on appetite and metabolism.

**Clinical Pearl / High-Yield Fact**
Fine tremors are a common side effect of lithium, but they can often be managed with dose adjustments or the addition of beta blockers. It is essential to monitor patients on lithium for signs of tremors and adjust the treatment plan accordingly.

**✓ Correct Answer: B. Fine tremors**