1. What is the primary pacemaker of the heart? A. Sinoatrial node B. Atrioventricular node C. Purkinje fibers D. Bundle of His Answer: A. Sinoatrial node Rationale: The sinoatrial node is responsible for initiating the electrical impulse that sets the rhythm of the heart, making it the primary pacemaker. 2. Which structure is responsible for the delay in electrical conduction, allowing for proper atrial contraction before ventricular contraction? A. Sinoatrial node B. Atrioventricular node C. Purkinje fibers D. Bundle of His Answer: B. Atrioventricular node Rationale: The atrioventricular node delays the electrical signal from the sinoatrial node, providing time for the atria to contract and fill the ventricles with blood before they contract. 3. What is the correct sequence of electrical conduction in the heart? A. AV node → SA node → Bundle of His → Purkinje fibers B. SA node → AV node → Bundle of His → Purkinje fibers C. Purkinje fibers → SA node → AV node → Bundle of His D. Bundle of His → Purkinje fibers → SA node → AV node Answer: B. SA node → AV node → Bundle of His → Purkinje fibers Rationale: The electrical impulse normally starts at the SA node, then travels to the AV node, through the Bundle of His, and finally through the Purkinje fibers to cause muscle contraction. Which of the following represents the correct placement of the electrodes for a standard 12-lead ECG? A. V1: 4th intercostal space, right sternal border B. V3: 5th intercostal space, midclavicular line C. V5: 6th intercostal space, midaxillary li