<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A 21-year-old man presents to a physician with repeated episodes of syncope and dizziness over the last month. On physical examination, his pulse is 64min while all other vital signs are normal. His 24-hour ECG monitoring suggests a diagnosis of sinus node dysfunction. His detailed genetic evaluation shows that he carries a copy of a mutated gene X that codes for an ion channel, which is the most important ion channel underlying the automaticity of the sinoatrial node. This is the first ion channel to be activated immediately after hyperpolarization. Which of the following ion channels does the gene X code for?? A: HCN-channels, B: L-type voltage-dependent calcium channels, C: T-type voltage-dependent calcium channels, D: Fast delayed rectifier IKr voltage-dependent K channels, E: Stretch-activated cationic channels,  <response>: This is a question about sinus node dysfunction in a young patient with genetic abnormalities. The key clues are the symptoms of syncope and dizziness, slow heart rate, and sinus node dysfunction on ECG monitoring. The question asks about identifying the ion channel encoded by the mutated gene X that underlies sinoatrial node automaticity. To answer this, I need to recall which ion channel is most important for sinoatrial node pacemaking and is activated immediately after hyperpolarization. The choices describe different types of ion channels, so I need to identify which one fits this description.A: HCN-channels 