A 54-Year-Old Woman Is Evaluated for Progressive Constipation, Anorexia, and a 5.4-Kg

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?

A) ATP
B) Ca²⁺
C) cAMP
D) GTP
E) IP₃

Correct Answer:

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