## Correct Answer: C. A fibers are more susceptible to pressure changes than C fibers This question tests understanding of differential nerve fiber susceptibility to mechanical compression. When an arm is slept upon, external pressure compresses the nerve trunk. The clinical presentation—**paresis without numbness**—is the key discriminator. Motor function (mediated by **A-alpha fibers**) is lost while sensory function (mediated by **C fibers** for pain/temperature) is preserved. This dissociation occurs because A fibers have larger diameter and higher metabolic demands; they are exquisitely sensitive to mechanical compression and ischemia from pressure. C fibers, being unmyelinated and smaller, are more resistant to pressure-induced conduction block. The mechanism: Pressure occludes blood supply and mechanically deforms the myelin sheath of large myelinated A fibers, causing conduction block within minutes. C fibers, lacking myelin and having lower conduction velocity, tolerate pressure better because their smaller cross-sectional area experiences less mechanical stress and their lower metabolic rate allows survival under hypoxic conditions longer. This is **pressure-induced neuropraxia**, a classic Indian medical school teaching scenario. The paresis resolves within hours as pressure is relieved and A-fiber conduction recovers. This is NOT axonal damage (which would cause numbness and prolonged recovery). The answer directly states A fibers are more susceptible to **pressure changes**—the operative mechanism here—not hypoxia alone. ## Why the other options are wrong **A. A fibers are more sensitive to hypoxia than B fibers** — While A fibers are indeed more metabolically active and hypoxia-sensitive than B fibers, this option misses the primary mechanism in pressure-induced nerve compression. The question specifically involves sleeping on the arm—a **mechanical pressure** scenario, not pure hypoxia. Additionally, B fibers (preganglionic autonomic) are not the relevant comparison; the clinical picture involves motor (A-alpha) vs. sensory (C) dissociation. This is a distractor that focuses on hypoxia when pressure is the dominant pathophysiology. **B. A fibers are more susceptible to hypoxia than C fibers** — Although A fibers ARE more hypoxia-sensitive due to higher metabolic demands and reliance on oxidative phosphorylation, this option again emphasizes hypoxia over the **mechanical pressure component** that is central to the clinical scenario. In pressure neuropraxia, the primary insult is mechanical deformation and local ischemia from compression, not systemic hypoxia. C fibers' resistance to hypoxia is true but irrelevant to the pressure-compression mechanism. This is an NBE trap using a true statement in the wrong context. **D. C fibers are more sensitive to pressure than A fibers** — This is the **direct opposite** of the correct answer and represents a fundamental misunderstanding of fiber physiology. C fibers are actually **resistant** to pressure because they are unmyelinated, smaller in diameter, and have lower metabolic demands. A fibers, being large and myelinated, are highly susceptible to pressure-induced conduction block. This option reverses the pathophysiology and would predict sensory loss (C-fiber dysfunction) with preserved motor function—the opposite of what is observed clinically. ## High-Yield Facts - **Pressure neuropraxia** causes motor loss (A-fiber block) with preserved sensation (C-fiber sparing) due to differential mechanical susceptibility. - **A fibers** (myelinated, large diameter) are exquisitely sensitive to compression; **C fibers** (unmyelinated, small) tolerate pressure better. - **Neuropraxia** is conduction block without axonal loss; recovery occurs within hours to days as pressure is relieved and myelin recovers. - **Metabolic demand hierarchy**: A-alpha > A-beta > A-delta > C fibers; higher demand = greater hypoxia sensitivity but also greater pressure sensitivity due to larger size. - **Clinical rule**: Paresis without numbness = motor fiber (A-alpha) block; numbness without paresis = sensory fiber (C/A-delta) block. ## Mnemonics **MACS Rule for Fiber Susceptibility** **M**otor (A-alpha) = **M**echanical pressure sensitive; **A**utonomic (B) = **A**noxia sensitive; **C** fibers = **C**an tolerate pressure. In compression injuries, motor goes first (paresis), sensation spared (no numbness). **BIG = VULNERABLE** **B**ig fibers (A-alpha, A-beta) are **V**ulnerable to **P**ressure because they are myelinated and metabolically demanding. **S**mall fibers (C) are **S**turdy against **P**ressure. ## NBE Trap NBE pairs hypoxia (options A and B) with pressure neuropraxia to trap students who conflate the two mechanisms. While A fibers ARE hypoxia-sensitive, the question's mechanical compression scenario makes **pressure susceptibility** the discriminating answer. Students who memorize "A fibers are hypoxia-sensitive" without understanding the context of pressure-induced neuropraxia will fall into this trap. ## Clinical Pearl In Indian clinical practice, **"Saturday night palsy"** (radial nerve compression from sleeping with arm over chair) and **"honeymoon palsy"** (ulnar nerve compression) are classic bedside examples of pressure neuropraxia. Patients present with isolated motor weakness (wrist drop, claw hand) without sensory loss—a pattern that immediately suggests A-fiber mechanical compression rather than C-fiber involvement. Recovery is complete within 2–3 weeks as the patient avoids the offending pressure. _Reference: Guyton & Hall Textbook of Medical Physiology Ch. 45 (Nerve and Muscle Physiology); Harrison Principles of Internal Medicine Ch. 379 (Peripheral Neuropathy)_
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