In this video, Dr. Webb takes the MCAT for the 1st time in 12 years.

Answers:
#1 The correct answer is A
Proteins have numerous functions in the body; they can be hormones (chemical messengers), enzymes (catalyze chemical reactions), structural proteins (physical support), transport proteins (carriers of important materials), and antibodies (which bind foreign particles). However, proteins cannot self-replicate; DNA is the only molecule capable of self-replication. Therefore, the correct answer for this question is choice (A).

#2: The correct answer is B
Normally, angiotensin II causes secretion of aldosterone from the adrenal cortex. Aldosterone serves to increase reabsorption of sodium, while promoting excretion of potassium and hydrogen ions. Thus, blocking the release of aldosterone should result in decreased reabsorption of sodium, while decreasing excretion of potassium and hydrogen ions. This eliminates choices (A) and (D) and makes choice (B) the correct answer. In the absence of aldosterone, less water reabsorption will occur, eliminating choice (C).

#3 The correct answer is B
Amides are defined as organic compounds in which an amine functionality is directly bonded to a carbonyl carbon. Choices (A) and (C) are both simple amines (primary and secondary); Choice (D) is a nitro compound, which is an organic compound with an -NO2 group. Only choice (B) has an amine group, -N(CH3)2, bonded to a carbonyl carbon, thereby creating the amide functional group-choice (B) is the correct answer.

#4: The correct answer is B
Neurulation occurs when the notochord causes differentiation of overlying ectoderm into the neural tube and neural crest cells. The neural tube ultimately becomes the central nervous system (brain and spinal cord), and neural crest cells migrate to other sites in the body to differentiate into a number of different tissues. Thus, only statement III is true.

#5 The correct answer is C
The process presented in the question stem is respiration; overall, in the presence of oxygen, a glucose molecule is broken down so as to give energy in the form of ATP, and two byproducts - carbon dioxide and water. Respiration begins in the cytoplasm but is completed in the mitochondria. The first step of respiration is glycolysis, which occurs in the cytoplasm. Pyruvate from this reaction is converted to acetyl CoA, which enters the Krebs cycle in the mitochondria. NADH and FADH2from the Krebs cycle enter the electron transport chain in the cristae, the inner membrane of the mitochondria. Therefore, the process described in the question stem is completed in the mitochondria, making choice (C) the correct answer.

#6 The correct answer is A
This question can be answered based on fundamental definitions. The rate-determining step is simply the slowest step in a proposed mechanism. Of the three steps shown, only step I is labeled as “slow” and must therefore be the bottleneck in the overall process. As such, it must be the rate-determining step, making choice (A) the correct answer.

#7 The correct answer is C
Healthy cells exhibit MHC class I molecules. Natural killer cells monitor the expression of MHC molecules on the surface of cells. Viral infection and cancer often cause a reduction in the expression of MHC class I molecules on the cell surface. Natural killer cells detect this lack of MHC and induce apoptosis in the affected cells.

#8 The correct answer is B
Generalization is the process by which similar stimuli can produce the same conditioned response. Here, the response to the taste and smell of lemons has generalized to that of all citrus.

#9 The correct answer is choice B. The number of genetically unique gametes produced by a given individual can be determined by using the formula “two to the nth power", where “n” is the number of heterozygous gene pairs that the individual has (23 = 8). Heterozygous pairs are those in which the two alleles are not the same. The number of homozygous gene pairs is not a factor in the formula because homozygous gene pairs do not contribute to the total number of possible variations in a gamete’s chromosome content. Why? Because all the gametes will have the same allele if the parent has two identical copies of the gene. If an organism has only two traits, and its genotype is big A little a, big B big B, there are only different types of gametes that it can produce: big A big B, and little a big B.

#10 The correct answer is A
Conversion disorder is marked by a motor or sensory symptom in the absence of an underlying physical or neurological cause. It is associated with an inciting event that, in this case, may have been the argument with her daughter. Her lack of concern over the deficit is referred to as la belle indifférence.

#11 Correct answer is B Four structural isomers exist for the formula C3H6Br2:

#12: D
#13: C
#14: D
#15: A
#16: C

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