Ans. D. 2:1 blockElectrocardiography and Electrophysiology of AV conduction blockAV conduction block typically is diagnosed electrocardiographically, which characterizes the severity of the conduction disturbances and allows one to draw inferences about the location of the block. AV conduction block manifests as slow conduction in its mildest forms and failure to conduct, either intermittent of persistently, in more severe varieties.a. First degree AV block (PR interval >200ms) is a slowing of conduction through the AV junction. The site of delay is typically in the AV node but may be in the atria, bundle of his, or his Purkinje system. A wide QRS is suggestive of delay in the AV node proper or less commonly in the bundle of His. In second degree AV block there is an intermittent failure of electrical impulse conduction from atrium to ventricle.b. Second degree AV block is sub classified as Mobitz type I (Wenckebach) or Mobitz type II. The periodic failure of conduction in Mobitz, type I block is characterized by a progressively lengthening PR interval, shortening of the RR interval, and a pause that is less than the two times the immediately preceding RR interval on the electrocardiogram (ECG). The ECG complex after the pause exhibits a shorter PR interval than that immediately preceding the pause. This ECG pattern most often arises because of decremental conduction of electrical impulses in the AV node.