**Core Concept**
Cholestyramine is a bile acid sequestrant used to lower cholesterol levels in the blood by binding to bile acids in the intestine, thereby reducing their reabsorption and increasing the liver's production of new bile acids from cholesterol. However, this process can have an unintended consequence in patients with hypertriglyceridemia.

**Why the Correct Answer is Right**
In patients with hypertriglyceridemia, the liver produces more very-low-density lipoprotein (VLDL) to compensate for the increased loss of cholesterol in the bile. Cholestyramine can further increase the production of VLDL by reducing the liver's uptake of cholesterol, leading to an increase in triglyceride levels. This is because the liver uses the cholesterol from the bile acids to synthesize VLDL, and with less cholesterol available, it produces more VLDL to transport the excess triglycerides. Therefore, cholestyramine is contraindicated in patients with hypertriglyceridemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Fibrates are actually used to treat hypertriglyceridemia by activating peroxisome proliferator-activated receptor-alpha (PPAR-alpha), which increases the breakdown of triglycerides in the liver and adipose tissue.
**Option B:** Simvastatin is a statin, which inhibits HMG-CoA reductase and reduces cholesterol production in the liver, thereby lowering LDL levels and having a neutral effect on triglycerides.
**Option C:** Niacin, or nicotinic acid, is used to treat hypertriglyceridemia by inhibiting the breakdown of triglycerides in adipose tissue and increasing the clearance of VLDL from the bloodstream.

**Clinical Pearl / High-Yield Fact**
When treating patients with hypertriglyceridemia, it's essential to choose medications that specifically target triglyceride metabolism, such as fibrates, rather than relying on general cholesterol-lowering agents that may worsen triglyceride levels.

**✓ Correct Answer: D. Cholestyramine**