25: Breaking Down Discrete MCAT Biology Questions

Session 25 In  this episode, Ryan and Bryan break down some fun, independent discrete biology questions on the MCAT. Good to know: There are 59 questions in each of the three different science sections - 15 are discrete while 44 questions are based on passages. Question #16: Which of the following will cause a blood pH 8.2? Decreased O2 concentration Decreased tidal volume Increased H2O concentration Increased respiratory rate Strategies for answering: Before looking at the answer choices, you should know that normal physiological blood pH is more like 7.35 - 7.4. So if the person has a blood pH of 8.2, that is wildly alkaline. So you're looking for an answer choice that is a higher pH, less acid or more base. Let's break down down the answer choices: (a) Decreased O2 concentration - This might be tempting knowing that O2 and CO2 have an inverse relationship in the blood. (b) Decreased tidal volume - Tidal volume is the natural resting breathing rate without forcibly trying to pull in or exhale more air. So if you breathe less, there is decreased tidal volume. This means that the CO2 building up in your body is not getting exhaled as well as it could since you're holding CO2 back in your body. For the purposes of blood pH, CO2 is acid. If you decreased the tidal volume, that would actually give you respiratory acidosis that would make your blood pH more acidic. So this is the opposite of what we're looking for. (c) Increased H2O concentration would not affect the pH because it would dilute the acid and the base at the same time. Adding more water won't  directly change the pH balance itself. (d) Increased respiratory rate - You start hyperventilating and blowing out a lot of CO2. By literally breathing acid out of your body, what's left behind in your blood is more basic so you're blood will become more basic and your blood pH could be higher. So this answer choice could tie more into the more basic blood pH. From the perspective of blood pH, CO2 is acid so you're exhaling acid. So if you stop exhaling, you hold your acid in while if you blow out a whole bunch, you blow the acid out and leaving base behind. Question #17: Patients with excess fat are more likely to require larger therapeutic doses of which vitamin: Vitamin B1 Vitamin C Vitamin D Vitamin B3 Strategies for answering: This questions is a classic example of "which one of these doesn't belong?" It almost doesn't matter what question is asked. Looking at all the choices, one thing jumps out right away. You have to know your water-soluble and your fat-soluble vitamins. Vitamins B and C are water-soluble and fat-soluble vitamins are Vitamins A, D, E and K. Hence, answer (c) Vitamin D is the only one that stands out. Question #29: Which of the following is most likely to use a protein channel to cross the eukaryotic cell membrane. Aldosterone Ca2+ ion Oxygen (O2) Carbon dioxide Strategies for answering: This is just like the question about vitamin solubility mentioned before. Aldosterone is a steroid hormone that can just diffuse right across the cell membrane and doesn't need a protein channel to get through a cell membrane. Gases (oxygen and carbon dioxide) can also just diffuse right across a cell membrane. They don't need a protein channel. Ions need a protein channel. Hence, answer (c) Calcium needs a protein channel to cross the membrane. Links and Other Resources: www.mededmedia.com www.specialtystories.com Next Step Test Prep www.nextstepmcat.com - Use the promo code: MCATPOD to save money on their products and services.

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