Vitamins, which come in many different types all of which are quite diverse in chemical configuration and function, can be any of several organic substances that are separated into water-soluble and fat-soluble groups. Originally defined as organic compounds obtainable in a normal diet and capable of maintaining life and promoting growth, vitamins are differrent from carbohydrates, fats, and proteins in function, as well as in the quantities in which organisms require them. So critical are vitamins to a body's essential strength and health that if they are absent from the diet or not properly absorbed by an organism, a specific deficiency disease may develop.
The term "vitamin" originated from "vitamine", a word first used in the early 19th century to designate a group of compounds considered vital for life (though the term "accessory food factor" sometimes is used interchangeably to refer to these substances). Like other nutrients, vitamin consumption is imperative to keep our bodies functioning properly, and if there is a lack of vitamin consumption, the body will fail to react in a way that's considered healthy. Lack of vitamin A will result in various disorders that most commonly involve the eye and the tissues around it. One of the earliest symptoms of vitamin A deficiency is nyctalopia, the medical term for night blindness, which causes a visual failure to adapt quickly from light to darkness and an inability to see in the dark. This aspect of vision is normally dependent on rhodopsin, a protein found in the eye that maintains itself only in the presence of vitamin A; in the lack of vitamin A, rhodopsin will malfunction.
These earlier symptoms are quite harmless but the side effects can become increasingly serious if not treated early on. If the deficiency is severe and persists, especially in malnourished infants and children, a condition known as xerophthahnia--when the eyes are sensitive to light, the secretion of lubricating tears is stopped, and the eyelids become swollen and develop pus--may develop. Furthermore, the mucous surfaces of the eye may become eroded, allowing infection to set in, thus leading to ulceration and other destructive changes of the cornea and other structures of the eye, resulting eventually in blindness.
Early signs of vitamin A deficiency may also be reflected in changes in the membranes of the mouth, throat, and respiratory and genitourinary passages where the lining membranes become malnourished and dry and lose their cilia, the tiny hairlike projections that normally help in clearing away foreign particles. The natural immune system is weakened and if insufficient intake of vitamin A is prolonged, the skin may become dry and rough. Vitamin A deficiency may also result in defective bone and teeth formation and in poor general growth.

单选题 According to paragraph 2, which of the following is true of vitamins? ______
A. Our body's operation is dependent on vitamins, which means that the body would not function without vitamins.
B. The term "vitamin" was first used in the 19th century, and until then, they were known as the "accessory food factor".
C. Insufficient consumption of vitamins may cause our body to stop functioning properly.
D. Lack of vitamin causes night blindness as well as many disorders that involve the eye and organs related to it.

单选题 The word "imperative" in paragraph 2 is closest in meaning to ______.
A. important B. vital C. unnecessary D. significant

单选题 Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
A. Vitamin A deficiency results in night blindness, which is also known as nyctalopia in medical terms.
B. The earliest sign of vitamin A deficiency is night blindness.
C. Among many disorders caused by lack of vitamin A, night blindness is the most common.
D. Vitamin A deficiency may cause many different types of ailments especially involving the eyes, including nyctalopia.

单选题 The phrase "this aspect of vision" refers to ______.
A. The adeptness to see clearly without depending on proteins.
B. The aptitude to rapidly adjust to the changes in the amount of light.
C. The ability to see clearly in complete darkness.
D. The competence to rapidly adjust from natural light to artificial light.

单选题 The author's description of symptoms of vitamin A deficiency mentions all of the following EXCEPT ______.
A. Skin tissues may lose moisture and become dry and chapped, decreasing the natural immunity.
B. Membranes in the mouth and throat may lose their cilia and allow more alien particles to enter the body than normal.
C. Lack of vitamin A may cause a person to have coarse skin, bad teeth, weak bones and insufficient natural immunity.
D. Bone and teeth may be affected negatively and when serious enough, may even stunt growth.

单选题 According to paragraph 4, which of the following is true of cilia?
A. Cilia are tiny extensions of membranes that normally provide help in clearing the membranes.
B. The term "cilia" refers to the hair lining of membranes that prevent foreign matter from entering our body.
C. The tiny projections in the lining membranes are known as cilia and they block incoming foreign matters.
D. Cilia are part of the lining membranes of the mouth and throat, and they help preventing external particles from entering our body.

单选题 The word "malnourished" in the passage is closest in meaning to ______.
A. emaciated B. melancholy C. ill D. hungry

单选题 Look at the four squares, , , and

单选题 Which of the following can be inferred from paragraph 5 about vitamin A? ______
A. Consistent intake of vitamin A is not recommended because it may accumulate in the liver.
B. It is not suitable for everyone because it may result in side effects such as nausea, loss of sight, dry skin etc. for some people.
C. It must be taken 150 milligrams or less at a time.
D. It must be taken with care because excessive or insufficient intake may result in illness.

单选题 The author uses nausea, blurred vision, and drowsiness as examples of ______.
A. the symptoms of vitamin A deficiency
B. the results of hypervitaminosis A
C. the results of storing toxic materials in the liver
D. the symptoms of excessive consumption of vitamins

单选题 The word "excessive" in the passage is closest in meaning to ______.
A. exclusive B. repeated
C. superfluous D. unreasonable

单选题 According to paragraph 6, which of the following is true of carotene? ______
A. They can be found in green vegetables.
B. They are found in all animal livers and dairy products.
C. They convert into vitamin A once consumed.
D. They are chemical substances human body needs in order to survive.

填空题 Directions: Complete the table below by indicating which of the answer choices describe the typical symptoms of insufficient intake of vitamin A and which describe the symptoms of excessive intake of vitamin A.
Symptoms of vitamin A deficiency
·______
Symptoms of vitamin A overdose
·______

A. The eyes become prone to irritation and cease to produce tears.
B. The patient may feel pain in the bones.
C. The respiratory organs may become infected and fail to operate properly.
D. Toxic particles may become accumulated within the liver.
E. The natural immunity of the human body may decrease and weaken.

F. The mouth and the throat may dry up and cease to produce saliva.

G. The patient may eventually become sightless.