The length of microvilli is about:

Correct Answer: D

Rationale: The correct answer is D: 1-2 microns. Microvilli are small, finger-like projections on the surface of cells that increase surface area for absorption. They are generally 1-2 microns in length. A is incorrect as 10-20 millimeters would be too long for microvilli. B is incorrect as 0.5-1.0 millimeters is also too long. C is incorrect as 20-50 microns is within the range of the length of cilia, not microvilli. Therefore, the correct answer is D based on the typical length of microvilli.

Correct Answer: D

Rationale: The correct answer is D because cells lining the sides of intestinal crypts of Lieberkühn are responsible for supplying replacement cells for the epithelium of villi. These cells undergo rapid division and differentiation to replace the cells shed from the villi, ensuring the continuous renewal and maintenance of the intestinal lining. This process is crucial for the absorption of nutrients and maintaining the integrity of the intestinal barrier. Choice A (secreting digestive enzyme) is incorrect because the cells in the intestinal crypts primarily function in cell replacement, not enzyme secretion. Choice B (secreting one of several various hormones) is incorrect as the main function of these cells is not hormone secretion but rather cell turnover. Choice C (secreting mucus) is incorrect as mucus secretion is typically associated with other specialized cells in the intestinal lining, such as goblet cells, not the cells in the crypts responsible for cell replacement.

Correct Answer: D

Rationale: Absorptive epithelial cells of the small intestine (enterocytes) are characterized by: A: Apical brush border composed of microvilli - This increases surface area for absorption. B: A glycocalyx containing various digestive enzymes - This aids in digestion and absorption. C: Junctional complexes including tight junctions separating apical from lateral cell membranes - This maintains cell polarity and regulates permeability. Therefore, choice D is correct as all these characteristics are typical of absorptive epithelial cells in the small intestine.

Correct Answer: B

Rationale: The correct answer is B: central vein. The hepatic lobule is a structural unit of the liver, consisting of hepatocytes arranged in a hexagonal pattern around a central vein. Hepatocytes receive oxygenated blood from the hepatic artery and nutrient-rich blood from the portal vein. These blood vessels merge within the lobule, and the blood flows towards the central vein. Bile produced by hepatocytes drains into bile canaliculi and eventually into bile ducts, which are not directly associated with defining the hepatic lobule. Therefore, the central vein is the correct answer as it is the vessel that receives blood from the sinusoids in the lobule.

Correct Answer: A

Rationale: The correct answer is A: hepatocytes. The term liver parenchyma refers to the functional cells of the liver responsible for its essential functions, such as metabolism, detoxification, and synthesis. Hepatocytes make up the majority of the liver's mass and are crucial for its normal functioning. Portal veins and hepatic arteries (choice B) are blood vessels supplying the liver, bile ducts (choice C) are responsible for transporting bile, and stored glycogen (choice D) is a form of stored energy in the liver, but they do not specifically refer to the liver parenchyma.