A 58-year-old man with a 15-year history of type 2 diabetes mellitus presents for neurological examination. When a vibrating tuning fork (128 Hz) is placed over his medial malleolus, he cannot perceive the vibration, though he can feel light touch and pain sensation normally. The structure marked **B** in the diagram is responsible for detecting this vibration stimulus. Which of the following pathological processes is most likely responsible for the loss of vibration sense in this patient?

Explanation

## Why option 1 is correct Diabetic peripheral neuropathy classically presents with early loss of vibration sense (large-fiber sensory loss) before pain and temperature sensation are affected. The structure marked **B** (Pacinian corpuscle) is a rapidly adapting, large-diameter mechanoreceptor located in deep dermal and subcutaneous tissues that detects high-frequency vibration (200–400 Hz). Loss of vibration sense at the medial malleolus using a 128 Hz tuning fork indicates dysfunction of both the Pacinian corpuscles AND the dorsal column–medial lemniscal pathway that transmits this information to the brain. In diabetic neuropathy, large myelinated fibers (Aα and Aβ) are preferentially affected early, causing selective loss of vibration and proprioception while pain/temperature (C and Aδ fibers) remain relatively preserved initially. This pattern is pathognomonic for large-fiber diabetic sensory neuropathy. (Guyton & Hall 14e Ch 47; standard diabetology teaching) ## Why each distractor is wrong - **Option 2**: Dorsal root ganglion degeneration affecting unmyelinated C-fiber function would cause loss of temperature and pain sensation first, not vibration. This describes small-fiber neuropathy, the opposite pattern from what this patient exhibits. - **Option 3**: Lateral corticospinal tract demyelination causes motor weakness and hyperreflexia (upper motor neuron signs), not isolated loss of vibration sense. The dorsal columns must be intact for vibration sense to be preserved. - **Option 4**: Acute inflammatory demyelination (e.g., Guillain–Barré syndrome) affects all fiber types relatively equally and presents acutely with ascending paralysis, not selective vibration loss in a chronic diabetic patient. **High-Yield:** Vibration sense loss at the medial malleolus = large-fiber neuropathy until proven otherwise; in diabetes, this is the earliest sensory sign and precedes pain/temperature loss. [cite: Guyton & Hall Textbook of Medical Physiology, 14th edition, Chapter 47: Somatic Sensations: I. General Organization, Receptor Physiology]