**Core Concept**
The question pertains to pharmacokinetic drug interactions involving lithium, a mood stabilizer used in the management of bipolar disorder. Lithium is primarily excreted by the kidneys, and its levels can be influenced by various factors, including other medications that affect renal function or lithium reabsorption.

**Why the Correct Answer is Right**
Lithium levels can be increased by non-steroidal anti-inflammatory drugs (NSAIDs), which inhibit prostaglandin synthesis and reduce renal blood flow, thereby decreasing lithium clearance. **Metformin**, a biguanide antidiabetic, can also increase lithium levels by competing with lithium for renal tubular secretion. **Methotrexate**, a chemotherapeutic agent, can decrease renal clearance of lithium, leading to increased levels. **Option C**, however, is not a correct answer as it is not a known drug interaction that increases lithium levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Not a known interaction that increases lithium levels.
**Option B:** Not a known interaction that increases lithium levels.
**Option D:** Not a known interaction that increases lithium levels.

**Clinical Pearl / High-Yield Fact**
It is essential to monitor lithium levels closely in patients taking NSAIDs, metformin, or other medications that may interact with lithium. A lithium level above 1.5 mEq/L can increase the risk of lithium toxicity.

**Correct Answer:** D.