Question:medium
Patient had SUN BURN, now takes bath in 40 degree water, and feels pain.
Patient had SUN BURN, now takes bath in 40 degree water, and feels pain.
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Pain from a NON-painful stimulus = allodynia; exaggerated pain from an already-painful stimulus = hyperalgesia. Is 40°C normally painful?
Updated On: Jun 22, 2026
- Thermal receptor: hyperalgesia
- Innocuous thermal receptor allodynia
- Thermal receptor allodynia
- Innocuous thermal receptor hyperalgesia
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The Correct Option is B
Solution and Explanation
Approach this by asking one question first: was the triggering stimulus normally painful or normally harmless?
Skin pain (nociceptive) thresholds for heat lie near 45$^{\circ}$C. A 40$^{\circ}$C bath is below that, so for healthy skin it is simply pleasantly warm - an innocuous thermal stimulus that should produce no pain at all.
Now layer on the pathology. Sunburn (UV injury) drives release of inflammatory mediators (prostaglandins, bradykinin, substance P) that sensitise peripheral nociceptors and lower their firing threshold. As a result the previously sub-threshold 40$^{\circ}$C warmth is now strong enough to fire these primary afferents and is consciously perceived as pain.
The textbook label for "pain produced by a stimulus that does not normally provoke pain" is allodynia. Contrast this with hyperalgesia, which is an amplified response to a stimulus that is already noxious - that label would only apply if, say, a clearly painful 50$^{\circ}$C contact hurt far more than expected.
So the precise description is a thermal, innocuous stimulus producing pain = innocuous thermal receptor allodynia. Options invoking hyperalgesia are wrong because the stimulus here is non-noxious, and the bare "thermal receptor allodynia" choice is less complete than the one that specifies the stimulus is innocuous.
Correct option: B.
Skin pain (nociceptive) thresholds for heat lie near 45$^{\circ}$C. A 40$^{\circ}$C bath is below that, so for healthy skin it is simply pleasantly warm - an innocuous thermal stimulus that should produce no pain at all.
Now layer on the pathology. Sunburn (UV injury) drives release of inflammatory mediators (prostaglandins, bradykinin, substance P) that sensitise peripheral nociceptors and lower their firing threshold. As a result the previously sub-threshold 40$^{\circ}$C warmth is now strong enough to fire these primary afferents and is consciously perceived as pain.
The textbook label for "pain produced by a stimulus that does not normally provoke pain" is allodynia. Contrast this with hyperalgesia, which is an amplified response to a stimulus that is already noxious - that label would only apply if, say, a clearly painful 50$^{\circ}$C contact hurt far more than expected.
So the precise description is a thermal, innocuous stimulus producing pain = innocuous thermal receptor allodynia. Options invoking hyperalgesia are wrong because the stimulus here is non-noxious, and the bare "thermal receptor allodynia" choice is less complete than the one that specifies the stimulus is innocuous.
Correct option: B.
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