**Core Concept**
Insulin therapy is a common treatment for diabetes mellitus, and it can cause several side effects due to its effects on carbohydrate and lipid metabolism. Insulin promotes glucose uptake in cells, inhibits gluconeogenesis, and increases glycogen synthesis.

**Why the Correct Answer is Right**
The correct answer is a side effect that does not occur with insulin therapy. Insulin can cause hypoglycemia, weight gain, and changes in lipid profiles. It can also cause lipodystrophy, particularly at injection sites. However, it does not typically cause **Option A:** (e.g. tachycardia), **Option B:** (e.g. increased blood pressure), or **Option C:** (e.g. nausea and vomiting) as direct side effects.

**Why Each Wrong Option is Incorrect**
* **Option A:** Tachycardia is not a direct side effect of insulin. However, hypoglycemia can cause tachycardia as a compensatory response.
* **Option B:** Increased blood pressure is not a typical side effect of insulin. However, weight gain and fluid retention can contribute to hypertension in some patients.
* **Option C:** Nausea and vomiting are not common side effects of insulin. However, they can occur in patients with diabetic gastroparesis or in response to hypoglycemia.

**Clinical Pearl / High-Yield Fact**
Insulin can cause lipodystrophy, particularly at injection sites, which can lead to lipoatrophy or lipohypertrophy. This is an important consideration for patients on long-term insulin therapy.

**Correct Answer:** A. Tachycardia