Q. A 25-year-old man comes to the clinic for a routine check-up. He mentions that he has been trying to gain weight by increasing his dietary intake. However, he noticed that despite increasing his intake of carbohydrates and proteins, he hasn't seen a significant weight increase. He expresses concern about his metabolism. The physician, interested in his case, decides to use it as a teaching opportunity for a group of medical students. He asks, "In the context of fatty acid synthesis, which of the following substances is a cofactor for the rate-limiting enzyme of the process that could potentially help our patient gain weight?"