**Core Concept**
The optimal ratio of carbohydrates in parenteral nutrition (PN) is a critical aspect of maintaining glucose homeostasis and preventing complications such as hyperglycemia and lactic acidosis. The choice of carbohydrate source and its proportion in the PN formulation is influenced by the patient's nutritional requirements, metabolic status, and underlying disease state.

**Why the Correct Answer is Right**
The recommended ratio of carbohydrates in PN is typically between 3:1 and 4:1 (carbohydrates:non-carbohydrates). This ratio is based on the concept of providing a balanced mix of energy sources to meet the patient's caloric needs while minimizing the risk of glucose intolerance and metabolic derangements. Carbohydrates should account for 60-70% of the total non-protein energy expenditure, with the remaining 30-40% provided by lipids.

**Why Each Wrong Option is Incorrect**
**Option A:** A ratio of 1:1 is too high in carbohydrates and may lead to hyperglycemia and insulin resistance, making it an inappropriate choice for parenteral alimentation.
**Option B:** A ratio of 5:1 is too low in carbohydrates and may not provide sufficient energy for the patient's nutritional needs, making it an inadequate choice.
**Option C:** A ratio of 2:1 may not provide a balanced mix of energy sources, increasing the risk of metabolic complications and making it an incorrect choice.

**Clinical Pearl / High-Yield Fact**
When initiating PN, it is essential to consider the patient's nutritional requirements, metabolic status, and underlying disease state to determine the optimal ratio of carbohydrates and other energy sources.

**Correct Answer:** C.