If you’re preparing for the Medical College Admission Test (MCAT), you are going to want to consult the experts. These selections from Kaplan’s MCAT Question of the Day series can help you sharpen your skills as you prepare to begin your potential journey into medical training.
The questions below come from three of the four MCAT sections—biological and biochemical foundations of living systems; chemical and physical foundations of biological systems; and psychological, social and biological foundations of behavior. A fourth section, critical analysis and reasoning skills (commonly referred to as CARS), is based largely on inference.
Medicine can be a career that is both challenging and highly rewarding but figuring out a medical school’s prerequisites and navigating the application process can be a challenge unto itself. For students preparing for medical school, the AMA premed glossary guide has the answers to frequently asked questions.
For those already in medical school, the AMA and Kaplan have teamed up to support you in reaching your goal of passing the USMLE® or COMLEX-USA®. If you're looking for additional resources, Kaplan provides free access to tools for pre-clinical studies, including Kaplan’s Lecture Notes series, Integrated Vignettes, Shelf Prep and more.
Section: Biological and biochemical foundations of living systems
Question: A double-stranded RNA genome isolated from a virus was found to contain 15% uracil. What percentage of guanine should exist in this virus's genome?
- 15%.
- 35%.
- 70%.
- 85%.
The correct answer is B.
Kaplan explains why: The percentage of uracil must equal that of adenine due to base-pairing. This accounts for 30% of the genome. The remaining 70% must be split evenly between guanine and cytosine, so they each account for 35% of the genome.
Section: Chemical and physical foundations of biological systems
Question: Many double-displacement reactions are enzyme-catalyzed via the “ping-pong” mechanism, so called because the reactants appear to bounce off the enzyme like a ping pong ball. These reactions typically have two reactants and two products. In a third-order reaction involving two reactants and two products, doubling the concentration of the first reactant causes the rate to increase by a factor of two. What will happen to the rate of this reaction if the concentration of the second reactant is cut in half?
- It will increase by a factor of two.
- It will increase by a factor of four.
- It will decrease by a factor of two.
- It will decrease by a factor of four.
The correct answer is D.
Kaplan explains why: Based on the information given in the question, the rate is first-order with respect to the concentration of the first reactant; when the concentration of that reactant doubles, the rate also doubles. Because the reaction is third-order, the sum of the exponents in the rate law must be equal to three. Therefore, the reaction order with respect to the other reactant must be 3 – 1 = 2. If the concentration of this second reactant is multiplied by 1/2, the rate will be multiplied by (1/2)2 = 1/4.
Section: Psychological, social, and biological foundations of behavior
Question: A patient comes in with a tumor of the pituitary gland, which grows upwards into the optic chiasm and causes a visual field defect. What is the most likely defect from compression of the optic chiasm?
- Complete blindness in one eye.
- Loss of the upper visual fields in both eyes.
- Loss of the nasal visual fields in both eyes.
- Loss of the temporal visual fields in both eyes.
The correct answer is D.
Kaplan explain why: The optic chiasm houses the crossing fibers from each optic nerve. Specifically, the fibers coming from the nasal half of the retina in each eye cross in the chiasm to join the optic tract on the opposite side. Remember that the lens of the eye causes inversion, so images on the nasal half of the retina actually originate in the temporal visual field. This condition is called bitemporal hemianopsia.
