The Question is from neet-zoology body-fluids-and-circulation regulation-of-cardiac-activity, and it was created by the author T HARITHA

Question

A patient presents with an abnormally high heart rate despite normal blood pressure and no signs of exertion. Which of the following is the LEAST likely explanation for this condition?

T HARITHA · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: Cardiac Conduction and Autonomic Nervous System Influence on Heart Rate
 **Related Topics**: Sympathetic Nervous System, Parasympathetic Nervous System, Sinoatrial (SA) Node, Atrioventricular (AV) Node, Catecholamines
 **Prerequisites**: Understanding of the autonomic nervous system's influence on heart rate, the role of the SA and AV nodes in heart rhythm.
## SOLUTION PATHWAY
**Given Information**:
  High heart rate (tachycardia)
  Normal blood pressure
  No exertion
**Method & Steps**:
 1. Analyze the effect of each option on heart rate.
  Reasoning:  We need to identify the option that would NOT lead to an increased heart rate.
 2. Evaluate each option individually.
  Working:
  A. Increased sympathetic stimulation of the SA node: The sympathetic nervous system releases norepinephrine, increasing the firing rate of the SA node, leading to tachycardia. This is a likely cause.
  B. Elevated levels of circulating catecholamines: Catecholamines (epinephrine and norepinephrine) increase heart rate and contractility. This is a likely cause.
  C. Increased parasympathetic stimulation of the SA node: The parasympathetic nervous system, via the vagus nerve, releases acetylcholine, which slows the heart rate. Increased parasympathetic stimulation would decrease, not increase, heart rate. This is the LEAST likely cause.
  D. A defect in the atrioventricular (AV) node conduction:  AV node defects can lead to heart blocks, slowing the heart rate or causing irregular rhythms.  However, some defects can also lead to faster heart rates (e.g., accelerated junctional rhythm), so this is still a possible, albeit less common, explanation for tachycardia than C.
 3. Determine the least likely cause.
  Result: Increased parasympathetic stimulation of the SA node (option C) is the least likely explanation for tachycardia (abnormally high heart rate).
  Verification:  The opposite effect (decreased heart rate) is expected from increased parasympathetic stimulation.
 ## OPTION ANALYSIS
A ✓: Increased sympathetic stimulation increases heart rate. 
B :x: Elevated catecholamines (like adrenaline) directly increase heart rate.
C :x: Increased parasympathetic stimulation (vagal tone) decreases heart rate, making it the least likely.
D :x: AV node defects can cause various arrhythmias, including some that could increase heart rate, but less directly than the other options.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing sympathetic and parasympathetic effects on heart rate: Remember sympathetic increases and parasympathetic decreases heart rate.
  2.Overlooking the role of catecholamines:  Understand that these hormones directly affect heart rate.
 **Quick Tips**:
  1.Use mnemonic devices to remember the effects of the autonomic nervous system on the heart (e.g.,  'Sympathetic speeds up, parasympathetic slows down').
  2.Visualize the anatomical pathways of the sympathetic and parasympathetic nervous systems to the heart.

T HARITHA · Answer

Increased sympathetic stimulation of the SA node

T HARITHA · Answer

Elevated levels of circulating catecholamines

T HARITHA · Answer

The correct Option is "Increased parasympathetic stimulation of the SA node"

T HARITHA · Answer

A defect in the atrioventricular (AV) node conduction

NEET Zoology Body Fluids And Circulation Regulation Of Cardiac Activity MCQs

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