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  1. (C) Nuclear receptor

    Thyroid hormones enter the cell and signal directly within the nucleus. This is distinct from steroid hormones, which bind a receptor in the cytosol and are then transported to the nucleus; and protein hormones such as parathyroid hormone, which bind receptors on the plasma membrane and induce signaling within the cell without actually entering the cell. Because thyroid hormones bind their receptor directly in the nucleus, none of the other answer choices describe a step in the thyroid signaling pathway. [Section 4.b]

  2. (A) Diphenhydramine blocks neuronal signaling more readily than loratadine

    First-generation antihistamines are nonpolar molecules, meaning they are not ionized and do not carry a charge, and they do not have a strong charge gradient. This means they can pass through cell membranes through diffusion relatively easily. Binding of antihistamines in the brain can lead to drowsiness, so first-generation antihistamines can have this side effect and are usually best taken at night before bed, when this will not be a concern. Second-generation antihistamines have a charged group on them, making them positively charged and unable to enter the brain through the blood brain barrier. None of the information contained in this question provides enough information to gauge the metabolism or effective dose of the drugs. [Section 3.d]

  3. (E) Luminal epithelial membrane

    Lactose intolerance is caused by loss of the lactase enzyme. Lactase is present on the brush border (part of the apical polarized plasma membrane) of the gut epithelium, where it processes lactose from milk and dairy products into glucose and galactose (smaller molecules that are then imported into the cell). It is a commonly tested enzyme since loss of lactase is fairly common. [Section 3.e]

  4. (C) Glial fibrillary acidic protein

    Glial fibrillary acidic protein, or GFAP, is an intermediate filament protein that is used to identify cell types in pathologic specimens. GFAP stains for astrocytes, which are one type of glial cells, or non-neuronal cells, in the brain (and the rest of the nervous system). This patient, who has an irregular mass in the temporal lobe on MRI, likely has developed a glioblastoma, but GFAP can also indicate less aggressive astrocytomas. [Section 4.e, Table 1-1]

  5. (F) Tubulin

    Several antimicrobial drugs target microtubule formation as their primary mechanism of action. Mebendazole is one example, used for parasitic infections (specifically, parasitic worms, known as “helminths”). These multicellular organisms rely on microtubule functions for many cellular functions, and the drugs are designed to specifically target parasite microtubules. [Section 4.f]

  6. (F) Endoplasmic reticulum

    Elevated liver enzymes indicate liver damage. Drug detoxification is one of the liver's major functions. In particular, the smooth endoplasmic reticulum is the site of detoxification, as well as steroid synthesis. This is in contrast to the rough endoplasmic reticulum, which classically is the site of protein synthesis. In this patient, hepatocytes are unable to cope with the amount of drug in the patient's system, ...

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