**Core Concept**
Insulin, a hormone produced by the pancreas, plays a crucial role in glucose regulation in the body. Its primary function is to facilitate the uptake of glucose by cells, thereby lowering blood glucose levels. However, insulin therapy can have several side effects, some of which are more significant than others.

**Why the Correct Answer is Right**
The most important side effect of insulin is **hypoglycemia**, a condition characterized by abnormally low blood glucose levels. This occurs when the administered insulin causes more glucose to be taken up by cells than is available from food or internal glucose production. Hypoglycemia can be severe, leading to confusion, loss of consciousness, and even death if not promptly treated. The mechanisms behind hypoglycemia involve the inhibition of glucose release from the liver and the stimulation of glucose uptake by peripheral tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Weight gain is a common side effect of insulin therapy, but it is not the most important one.
**Option B:** Allergic reactions to insulin are rare and not typically life-threatening.
**Option C:** Fluid retention is a possible side effect of insulin, but it is not as critical as hypoglycemia.

**Clinical Pearl / High-Yield Fact**
The risk of hypoglycemia can be minimized by taking insulin at regular intervals, monitoring blood glucose levels, and adjusting the dose of insulin based on the results.

**Correct Answer:** D. Hypoglycemia.