**Core Concept**
Lithium is a narrow therapeutic index medication used in the treatment of bipolar disorder. Its therapeutic range is narrow, and levels above 1.25 mEq/L can lead to toxicity. Lithium exerts its therapeutic effects by inhibiting the reuptake of serotonin and norepinephrine, and also has a neuroprotective effect.

**Why the Correct Answer is Right**
At a lithium level of 2.3 mEq/L, the patient is at risk of developing lithium toxicity. One of the common manifestations of lithium toxicity is **neuromuscular excitability**, which can manifest as muscle weakness, tremors, and **ataxia**. This is due to the inhibition of the sodium-potassium pump, leading to a decrease in the sodium gradient across the neuronal membrane, and an increase in the intracellular concentration of sodium. This can lead to an increase in the activity of the nervous system, resulting in tremors and muscle weakness.

**Why Each Wrong Option is Incorrect**
**Option A:** Nephrogenic diabetes insipidus is a side effect of lithium, but it is not the most likely side effect at a lithium level of 2.3 mEq/L.
**Option B:** Weight gain is a common side effect of lithium, but it is not the most likely side effect at this level of toxicity.
**Option C:** **Tremors** are a common side effect of lithium, but they are not the most specific side effect at a lithium level of 2.3 mEq/L.

**Clinical Pearl / High-Yield Fact**
Lithium levels should be monitored regularly in patients on long-term lithium therapy. A lithium level of 2.3 mEq/L is considered toxic and requires immediate medical attention.

**Correct Answer: C.** Tremors