challenge: What is the primary function of the AP-3 protein complex, which includes the protein encoded by the AP3B1 gene?
  
  A. Cell signaling and transcriptional regulation
  B. Regulation of protein synthesis in the endoplasmic reticulum
  C. Organelle biogenesis associated with melanosomes, platelet dense granules, and lysosomes
  D. Maintenance of cytoskeletal structure and dynamics
  Answer: 