I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
Q- A 34-year-old man comes to the office because of a 1-month history of diarrhea. He has a history of pheochromocytoma treated 2 years ago. His mother is being treated for a tumor of her parathyroid gland. He has no other history of major medical illness and takes no medications. His temperature is 37.0°C (98.6°F), pulse is 84/min, respirations are 10/min, and blood pressure is 120/75 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 97%. Vital signs are within normal limits. Physical examination shows a 3-cm, palpable mass on the right side of the neck. A biopsy specimen of the mass shows a neuroendocrine neoplasm of parafollicular cell origin. The most likely cause of the findings in this patient is a mutation in which of the following types of genes?
(A) Cell cycle regulation gene
(B) DNA mismatch repair gene
(C) Metastasis suppressor gene
(D) Proto-oncogene
(E) Tumor suppressor gene
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Answer
This patient’s history of diarrhea, pheochromocytoma and his mother’s history of parathyroid tumor suggests the diagnosis of multiple endocrine neoplasia type 2A (MEN2A). It is a hereditary syndrome caused by mutations in the RET proto-oncogene. It is associated with medullary thyroid carcinoma, pheochromocytoma and parathyroid hyperplasia or adenoma. Diarrhea in this patient is mainly caused by secretion of calcitonin from the medullary thyroid carcinoma.
The correct answer is D