Free Online USMLE Step 1 Questions and Answers 3


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10. A 54-year-old woman is evaluated for progressive constipation, anorexia, and a 5.4-kg (12-lb) weight loss over the past several months. Physical examination is unremarkable. Stool guaiac test is positive, and a colonoscopy is performed. An exophytic mass is identified in the sigmoid colon. The patient undergoes a left hemicolectomy, and histopathology of the surgical specimen is positive for adenocarcinoma. Molecular testing of the cancer cells reveals a mutation in the KRAS gene that results in constitutive activation of the Ras protein. Under normal circumstances, this protein is only active when bound to which of the following substances?

Usmle3 3

Correct Answer: D

Answer Explanation:

(Choices A and C) ATP serves as a phosphate source for kinase-dependent phosphorylation reactions involved in numerous intracellular signaling pathways. It is also used as a substrate to produce cyclic AMP (cAMP) in the cAMP second messenger system.
(Choice B) Ca2+ is important for cell signaling as its intracellular presence alters the function of many proteins and enzymes. Ca2+ is required for many basic physiologic activities including apoptosis, muscular contraction, and neuronal transmission.
(Choice E) IP3 is the water-soluble component of the IP3/JDAG second messenger system. IP3 acts on the endoplasmic reticulum to cause intracellular Ca2+ release while DAG remains in the cell membrane and activates membrane-bound protein kinases.

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