## Distinguishing MEN 2A from MEN 2B ### Clinical Context Both MEN 2A and MEN 2B are caused by **activating RET mutations**, but they have distinct phenotypes. The presence of mucosal neuromas and marfanoid features is pathognomonic for MEN 2B and absent in MEN 2A. ### Comparative Table | Feature | MEN 2A | MEN 2B | |---------|--------|--------| | **RET mutation** | Cys residues (exons 10, 11) | Met918Thr (exon 16) | | **Medullary thyroid cancer** | 100% (age 20–50) | 100% (earlier onset, age 10–20) | | **Pheochromocytoma** | 50% | 50% | | **Primary hyperparathyroidism** | 20–30% | Rare/absent | | **Mucosal neuromas** | Absent | 100% (lips, tongue, GI tract) | | **Marfanoid habitus** | Absent | Present (tall, slender, joint laxity) | | **Intestinal ganglioneuromatosis** | Absent | 100% (constipation, diarrhea) | | **Corneal nerve thickening** | Absent | Present | **High-Yield:** Mucosal neuromas + marfanoid habitus = MEN 2B. Their absence = MEN 2A. **Mnemonic:** **MEN 2B = "Bumpy"** — **B**umpy lips (mucosal neuromas), **B**one abnormalities (marfanoid), **B**owel dysfunction (ganglioneuromatosis). ### Clinical Pearl **Clinical Pearl:** MEN 2B patients often present with mucosal neuromas visible on oral examination before thyroid cancer is diagnosed. The neuromas are small, firm, flesh-colored papules on the lips, tongue, and buccal mucosa. This physical finding is virtually diagnostic and should prompt RET testing and aggressive thyroid surveillance. ### Why This Discriminates Mucosal neuromas and marfanoid habitus are: - **Visible on examination** — no imaging or biochemistry needed - **100% present in MEN 2B** — nearly universal - **Completely absent in MEN 2A** — never seen - **Pathognomonic** — not found in other genetic syndromes [cite:Harrison 21e Ch 405] 
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