## Why Option 1 is correct The clinical anchor from Robbins 10e is that recurrent aphthous stomatitis (RAS) characteristically affects NON-keratinized mucosa (lip, cheek, floor of mouth, ventral tongue, soft palate) while SPARING keratinized areas (gingiva, hard palate, dorsum of tongue). This anatomical distribution is the key distinguishing feature from HSV infection, which affects both keratinized and non-keratinized mucosa indiscriminately. The round ulcer with grayish-white base marked **A** in the diagram, when located on non-keratinized surfaces, is pathognomonic for RAS and helps differentiate it from HSV-related ulceration. ## Why each distractor is wrong - **Option 2**: Both aphthous ulcers and HSV ulcers can present without preceding vesicles; this is not a reliable distinguishing feature. HSV typically begins with vesicles that rupture into ulcers, but recurrent HSV may present directly as erosions without visible vesicles. - **Option 3**: This is factually incorrect. Aphthous ulcers are NOT confined to lips and perioral region—they occur on non-keratinized mucosa including cheek, floor of mouth, and soft palate. HSV can also affect the lips and perioral region, making this distinction unreliable. - **Option 4**: This describes the herpetiform variant of RAS (multiple tiny 1–3 mm lesions), which is only one of three clinical types (~10% of cases). Minor aphthous ulcers are typically single or few lesions <1 cm. This option conflates subtypes and is not the primary distinguishing feature. **High-Yield:** RAS = non-keratinized mucosa; HSV = both keratinized + non-keratinized—this anatomical distinction is the gold standard for clinical differentiation without biopsy. [cite: Robbins and Cotran Pathologic Basis of Disease, 10th Edition, Chapter 17]
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