The primary cause of dentin hypersensitivity in patients is the loss of enamel leading to exposed dentin tubules. Enamel serves as a protective layer for the underlying dentin, which contains microscopic tubules filled with fluid and nerve endings. When enamel is worn away due to factors like abrasion, erosion, or gum recession, these dentin tubules become exposed. External stimuli such as thermal changes, tactile pressure, or chemical irritants can then stimulate the nerve endings within these tubules, resulting in the sharp, transient pain characteristic of dentin hypersensitivity.

It is important to distinguish this from other conditions:
- Inflammation of the dental pulp (Option 2) typically causes more severe, lingering pain and is associated with pulpitis, not hypersensitivity.
- Accumulation of dental plaque (Option 3) can lead to caries and gingivitis but does not directly cause dentin hypersensitivity.
- Caries-induced destruction of enamel alone (Option 4) involves decay but does not necessarily expose dentin tubules unless the lesion progresses significantly.

Therefore, the key factor in dentin hypersensitivity is the direct exposure of dentinal tubules due to enamel loss, which allows external stimuli to trigger nerve responses.

Reference: Sturdevant's Art and Science of Operative Dentistry, Volume 1, Chapter 4, Page 120