**Core Concept**
Lithium is a narrow therapeutic index medication used in the treatment of bipolar disorder. Its therapeutic window is between 0.6-1.25 mEq/L, and levels above this range can lead to toxicity.

**Why the Correct Answer is Right**
At a lithium level of 2.3 mEq/L, the patient is at risk of developing lithium toxicity. This is because the kidney's ability to excrete lithium is impaired at high concentrations, leading to a buildup of lithium in the body. Lithium toxicity can cause a range of symptoms, including tremors, confusion, ataxia, and in severe cases, seizures and coma. The kidneys are especially affected, with symptoms such as nephrogenic diabetes insipidus, where the patient experiences excessive thirst and polyuria.

**Why Each Wrong Option is Incorrect**
**Option A:** Polyphagia is not a typical side effect of lithium toxicity. Instead, patients may experience polydipsia due to nephrogenic diabetes insipidus.

**Option B:** Confusion and ataxia are indeed symptoms of lithium toxicity, but they are not the most specific or severe side effect.

**Option C:** Weight gain is a common side effect of lithium, but it is not the most likely side effect in a case of lithium toxicity.

**Option D:** Hypotension is not typically associated with lithium toxicity. In fact, lithium can cause hypertension due to its effects on the kidneys and vasculature.

**Clinical Pearl / High-Yield Fact**
Remember the "Lithium Toxicity" mnemonic: "Loose stools, tremors, confusion, ataxia, nausea, vomiting, seizures, and coma" - a classic sequence of symptoms that can occur in patients with lithium toxicity.

**Correct Answer: B. Confusion and ataxia are common symptoms of lithium toxicity.**