MC4R deficiency is identified by a mutation in which gene?

The identification of MC4R deficiency is linked directly to a mutation in the MC4R gene. The melanocortin-4 receptor, encoded by the MC4R gene, plays a significant role in regulating appetite and energy homeostasis. When there is a mutation in this gene, it can lead to disruptions in this pathway, which is associated with obesity due to increased appetite and reduced energy expenditure.

Other genes listed, such as ALMS1, ADRB3, and LEP, are associated with different pathways in energy metabolism and obesity but do not directly involve the MC4R pathways. For instance, the LEP gene encodes leptin, a hormone involved in regulating energy balance, while mutations in the ADRB3 gene are associated with adipocyte regulation. Similarly, ALMS1 mutations relate to Alström syndrome, which includes obesity among other symptoms but does not implicate the MC4R directly.

Thus, the correct answer is based solely on the specific mutation represented in the MC4R gene, directly linking its dysfunction to the observed phenotype of obesity in individuals with MC4R deficiency.