**Core Concept**
Thiazolidinediones (TZDs) are a class of medications used in the treatment of type 2 diabetes mellitus. They work by activating peroxisome proliferator-activated receptor gamma (PPAR-γ), which regulates glucose and lipid metabolism. This activation leads to improved insulin sensitivity and decreased glucose production in the liver.

**Why the Correct Answer is Right**
The activation of PPAR-γ by thiazolidinediones results in increased expression of genes involved in glucose and lipid metabolism. One of the common side effects of TZDs is fluid retention, which occurs due to increased sodium retention in the kidneys. This is caused by the activation of the sodium-retaining enzyme, Na+/K+-ATPase, in the collecting ducts of the kidneys. The increased fluid retention can lead to edema, weight gain, and exacerbation of heart failure.

**Why Each Wrong Option is Incorrect**
**Option A:** Although weight gain is a potential side effect of thiazolidinediones, it is not the most common side effect. Weight gain occurs due to increased appetite and increased glucose storage in adipose tissue, but it is not as frequent as fluid retention.
**Option B:** Hypoglycemia is not a common side effect of thiazolidinediones. In fact, thiazolidinediones can cause hyperglycemia as a side effect due to increased glucose production in the liver.
**Option C:** Thiazolidinediones can cause an increase in LDL cholesterol as a side effect, but this is not as common as fluid retention.

**Clinical Pearl / High-Yield Fact**
When prescribing thiazolidinediones, it is essential to monitor patients for signs of fluid retention, such as edema and weight gain. Patients with a history of heart failure should be closely monitored, as thiazolidinediones can exacerbate heart failure.

**Correct Answer: C. Increased LDL cholesterol.**