**Core Concept**
The human body converts carbohydrates into glucose, which is then used by cells for energy production. When glucose is in excess, it can be converted into fatty acids and ultimately triglycerides through the process of de novo lipogenesis. This process is essential for the synthesis of triglycerides in the liver.

**Why the Correct Answer is Right**
The patient's switch to a high-carbohydrate diet has led to an increase in his triglyceride levels. This is due to the increased rate of de novo lipogenesis, where excess glucose is converted into fatty acids and subsequently into triglycerides. The liver plays a crucial role in this process, and the increased availability of glucose from the high-carbohydrate diet has stimulated this pathway. The end result is an increase in triglyceride production and secretion into the bloodstream, leading to elevated triglyceride levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Phospholipids are a type of lipid that is not directly involved in the increase in triglyceride levels. They are an important component of cell membranes and are not produced in excess through de novo lipogenesis.

**Option B:** Triglycerides are the very molecules whose levels the patient is trying to reduce, not increase. The question is asking about the component of the diet that is causing an increase in triglyceride levels, not the triglycerides themselves.

**Option C:** Amino acids are the building blocks of proteins, not triglycerides. While a low-protein diet might seem beneficial for reducing triglyceride levels, the increase in triglyceride levels is not caused by the amino acids in the diet.

**Clinical Pearl / High-Yield Fact**
De novo lipogenesis is a critical pathway for the synthesis of triglycerides in the liver. This process is stimulated by excess glucose availability, which can occur with high-carbohydrate diets. It is essential to note that this pathway is also influenced by other factors, such as insulin resistance and genetic predisposition.

**Correct Answer:**
✓ Correct Answer: D. Carbohydrates