**Core Concept**
Lithium is a mood stabilizer used in the treatment of mania, and its use is associated with several potential side effects and interactions. Monitoring serum levels is crucial to prevent toxicity.

**Why the Correct Answer is Right**
The commonest neurological side effect of lithium is tremor, which is thought to be related to its effect on the cerebellum and the neurotransmitter glycine. Lithium inhibits the reuptake of glycine, leading to increased glycine levels in the cerebellum, which can cause tremors. Propranolol, a beta-blocker, is often used to treat lithium-induced tremor. Amiloride is used to treat lithium-induced nephrogenic diabetes insipidus, a condition where the kidneys are unable to concentrate urine due to lithium's effect on the renal tubules.

**Why Each Wrong Option is Incorrect**
* **Option A:** Tremor is not the only neurological side effect of lithium, but it is one of the most common. Other side effects include ataxia, confusion, and seizures.
* **Option B:** Toxic levels of lithium are generally considered to be above 1.5 mmol/L (not mg/dl), which can cause severe neurological symptoms and even death.
* **Option C:** Amiloride is indeed used to treat lithium-induced nephrogenic diabetes insipidus, but it is not the drug of choice (DOC) for this condition.
* **Option D:** Lithium is not 90% protein bound; it is approximately 30% bound to plasma proteins.

**Clinical Pearl / High-Yield Fact**
Lithium levels should be monitored regularly, and the dose should be adjusted accordingly to prevent toxicity. Patients on lithium should also be advised to drink plenty of water to prevent dehydration.

**Correct Answer:** E. Tremor is treated with propranolol.