Infective organism causing AV block
**Core Concept**
Acute infective myocarditis can cause atrioventricular (AV) block by inflaming the AV node and surrounding tissues, disrupting the normal electrical conduction pathway between the atria and ventricles.
**Why the Correct Answer is Right**
The AV node is richly innervated with autonomic nerve fibers, particularly parasympathetic fibers that release acetylcholine, which can slow AV conduction. In infective myocarditis, the inflammation of the AV node can lead to fibrosis and scarring, causing a permanent or transient block in electrical conduction. This is often seen in cases of Lyme disease, diphtheria, or typhoid fever.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify an infective cause of AV block. While non-infective causes like ischemia or fibrosis can also lead to AV block, the question specifically asks for an infective organism.
**Option B:** This option is incorrect as it refers to a non-infective cause of AV block, specifically a congenital condition. While this can lead to AV block, it is not an infective organism.
**Option C:** This option is incorrect as it refers to a non-infective cause of AV block, specifically a medication side effect. While certain medications can cause AV block, this is not an infective organism.
**Option D:** This option is incorrect as it refers to a non-infective cause of AV block, specifically a structural heart defect. While this can lead to AV block, it is not an infective organism.
**Clinical Pearl / High-Yield Fact**
In cases of suspected AV block due to infective myocarditis, it is essential to investigate for underlying infections, particularly Lyme disease, diphtheria, or typhoid fever, and to consider the need for antibiotic treatment.
**Correct Answer:** B. Lyme disease.