MCAT - Biological Science Practice

The most widely-used differential stain for bacteria is:

All of the following are true statements about the brain EXCEPT:

Microfilaments and microtubules form a complex network of fibers within the eukaryotic cell that can be disassembled and reassembled. If this were to occur, what is likely to be the immediate result?

The small intestine is one of many organs that has endocrine functions in addition to its more familiar roles in the body. By producing such hormones as intestinal gastrin, gastric inhibitory peptide (GIP), secretin, and cholecystokinin, the small intestine regulates the activities of neighboring digestive organs and their secretions. The thymus gland secretes thymosin, a protein hormone that stimulates the maturation and differentiation of T-lymphocytes. These cells of the immune system provide cell-mediated immunity, which not only includes attacks on infectious agents, but on infected cells as well. Other endocrine-producing structures include the pineal gland, the kidney, and the heart. In response to light stimuli entering the eye, the pineal gland (located in the brain) releases melatonin, which is believed to play a role in regulating daily circadian rhythms. The kidney regulates red blood cell production through the secretion of erythropoietin. The heart, by secreting atrial natriuretic factor (ANF), helps regulate blood pressure, salt, and water balance. Thymosin can directly _____.

Phenylalanine is an amino acid that is necessary in small amounts for proper human nutrition. The normal metabolism of this substance requires the enzyme phenylalanine hydroxylase. The ability to produce the enzyme is controlled by an autosomal dominant trait. Approximately one in 15,000 whites in the United States is born without the capacity to produce the enzyme due to a homozygous recessive condition called phenylketonuria (PKU). In people with PKU, phenylalanine accumulates in the blood, and phenylpyruvic acid accumulates in the urine-giving it a distinctive odor. High levels of phenylalanine in the brain and spinal fluid during the first weeks after birth causes mental retardation in PKU individuals. Since PKU can be identified by a blood test at birth, dietary treatment can be administered to prevent this damage. If placed on a 1owphenylalanine diet for the first few years of life, symptoms can be avoided. A woman, homozygous for PKU, was successfully treated after birth to avoid symptoms of the disorder. If she becomes pregnant, why must similar precautions be taken to control her diet again?