**Core Concept**
Metformin is a biguanide antidiabetic medication that works by decreasing hepatic glucose production and increasing insulin sensitivity. The key difference between metformin and phenformin lies in their pharmacokinetic and pharmacodynamic profiles.

**Why the Correct Answer is Right**
Metformin is preferred over phenformin due to its lower risk of lactic acidosis. Phenformin is metabolized to a lesser extent, resulting in higher levels of the active metabolite, which is associated with an increased risk of lactic acidosis. Metformin, on the other hand, is primarily excreted unchanged in the urine, reducing the risk of this serious side effect.

**Why Each Wrong Option is Incorrect**
**Option A:** Metformin has a faster onset of action compared to phenformin. **Incorrect because** both medications have a similar onset of action, and this is not the primary reason for preferring metformin.

**Option B:** Metformin has a more favorable effect on lipid profiles compared to phenformin. **Incorrect because** both medications have similar effects on lipid profiles, and this is not a primary consideration for choosing metformin.

**Option C:** Metformin has a lower risk of gastrointestinal side effects compared to phenformin. **Incorrect because** both medications can cause gastrointestinal side effects, but this is not the primary reason for preferring metformin.

**Option D:** Metformin is more effective in reducing HbA1c levels compared to phenformin. **Incorrect because** both medications have similar effects on HbA1c levels, and this is not the primary reason for preferring metformin.

**Clinical Pearl / High-Yield Fact**
Metformin is the preferred biguanide due to its lower risk of lactic acidosis, which is a serious and potentially life-threatening side effect. This is a critical consideration when choosing an antidiabetic medication, especially in patients with renal impairment or other risk factors.

**Correct Answer:** C. Lower risk of lactic acidosis.