L:\usmle review 7 - Sinoe medical homepage.

33. The correct answer is D. The tumor described is a craniopharyngioma, which is one of the
more common brain tumors of children. These tumors arise from epithelial rests derived from
Rathke’s pouch, which is an oral invagination that gives rise to the cells that form the anterior
pituitary gland. Histologically, craniopharyngiomas can resemble ameloblastomas, which are
tumors derived from dental epithelium.
Note that this question could also have been answered very simply by noting that the
hypothalamus, pineal gland, pituitary gland, and the superior colliculus are all adult structures;
only Rathke’s pouch is an embryonic structure. Therefore, only Rathke’s pouch could be the
source of epithelial rests, which are remnants of embryonic tissues that persist in the adult.
Although craniopharyngiomas often occur near the hypothalamus (choice A) and posterior
portion of the pituitary gland (choice C), they do not arise from neural tissue.
The pineal gland (choice B) and superior colliculus (choice E) are found on the posterior aspect
of the brain stem.
34. A new antifungal medication is being tested in Phase I clinical trials. Examination of the
pharmacokinetic properties of the drug reveals that the half-life of the drug is 6 hours. If a
continuous intravenous infusion of this drug were started on a research subject, how long would it
take to reach 75% of steady state
(A)
(B)
(C)
(D)
(E)
(F)
3 hours
6 hours
9 hours
12 hours
18 hours
24 hours
34. The correct answer is D. The rule of thumb is that the plasma concentration will reach 50%
in one half-life, 75% in two half-lives, 87.5% in three half-lives, etc., so that the difference