Correct Answer: A. Medullary thyroid carcinoma
Medullary thyroid carcinoma (MTC) is a neuroendocrine malignancy arising from parafollicular C cells of the thyroid. The pathognomonic histological finding is amyloid deposition, which represents amyloid-A protein derived from calcitonin and its precursor procalcitonin. This amyloid is Congo red-positive and shows apple-green birefringence under polarized light—a diagnostic hallmark. The clinical presentation of neck mass with odynophagia and dyspnea reflects local invasion and mass effect. MTC accounts for 3–5% of thyroid cancers in India. It secretes excessive calcitonin (diagnostic marker: serum calcitonin >100 pg/mL), causing systemic symptoms. Importantly, MTC is associated with MEN 2A and MEN 2B syndromes in ~25% of cases (RET proto-oncogene mutations), making genetic screening essential in Indian families. The amyloid deposition distinguishes MTC from all other thyroid malignancies, which do not produce calcitonin-derived amyloid. Treatment involves total thyroidectomy with central neck dissection; prognosis depends on stage and calcitonin doubling time.
Why the other options are wrong
B. Papillary thyroid carcinoma — Papillary carcinoma is the most common thyroid malignancy (80% of cases) but does NOT produce amyloid. Histology shows papillary architecture with Orphan Annie eye nuclei and psammoma bodies—not amyloid deposition. This is a classic NBE trap: students may confuse 'thyroid cancer + neck mass' with papillary carcinoma, ignoring the amyloid clue. C. Anaplastic thyroid carcinoma — Anaplastic carcinoma is highly aggressive and presents with rapid dyspnea and dysphagia, mimicking the clinical picture. However, histology shows spindle cells, giant cells, and necrosis—NOT amyloid. Amyloid deposition is not a feature of anaplastic carcinoma, making this a clinical red herring. D. Follicular thyroid carcinoma — Follicular carcinoma presents as a thyroid nodule but lacks amyloid deposition. Histology shows follicular architecture with capsular/vascular invasion. It does not produce calcitonin or calcitonin-derived amyloid, eliminating this option despite similar epidemiology to other differentiated thyroid cancers.
High-Yield Facts
- Amyloid deposition (Congo red-positive, apple-green birefringence) is pathognomonic for medullary thyroid carcinoma and derived from calcitonin/procalcitonin.
- Serum calcitonin >100 pg/mL is the diagnostic marker for MTC; calcitonin doubling time predicts prognosis.
- MEN 2A/2B association occurs in ~25% of MTC cases due to RET proto-oncogene mutations; genetic screening is mandatory in Indian families.
- Parafollicular C-cell origin distinguishes MTC from follicular-derived thyroid cancers (papillary, follicular, anaplastic).
- Total thyroidectomy + central neck dissection is the standard surgical treatment; external beam radiotherapy for advanced/recurrent disease.
Mnemonics
MTC = Calcitonin + Amyloid Medullary = C-cell (parafollicular) → Calcitonin production → Amyloid deposition. Remember: MTC is the ONLY thyroid cancer that makes amyloid. MEN 2 + MTC Rule MTC is part of MEN 2A (medullary thyroid + pheochromocytoma + hyperparathyroidism) and MEN 2B (medullary thyroid + pheochromocytoma + mucosal neuromas). Always screen RET gene in MTC patients.
NBE Trap
NBE pairs 'thyroid cancer + neck mass + dyspnea' with papillary carcinoma (the most common type) to lure students away from the amyloid clue. The discriminating histological finding (amyloid) is the key to avoiding this trap.
Clinical Pearl
In Indian practice, any thyroid cancer patient with elevated serum calcitonin and amyloid on biopsy should trigger immediate RET genetic testing and screening of first-degree relatives for MEN 2 syndrome—early identification of RET mutations allows prophylactic thyroidectomy in children, dramatically improving survival.
_Reference: Robbins Ch. 8 (Endocrine System); Harrison Ch. 375 (Thyroid Malignancy)_