Breaking Down Discrete MCAT Biology Questions

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Breaking Down Discrete MCAT Biology Questions

Session 25

In this episode of The MCAT Podcast, we break down some fun, discrete MCAT biology questions.

MCAT Biology Questions: Discrete vs Passage-Based

There are 59 questions in each of the three different science sections of the MCAT. In each section, 15 questions are discrete while 44 questions are based on passages. Today we’re looking at discrete MCAT biology questions.

[Related episode: MCAT Foundational Topic: Biology Questions Broken Down.]

Question #16: Which of the following will cause a blood pH 8.2?

Answer choices:

  • (A) Decreased O2 concentration
  • (B) Decreased tidal volume
  • (C) Increased H2O concentration
  • (D) 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 or 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 would cause this higher pH in the blood, which is less acidic or more basic.

Let’s break down down the answer choices:

(A) Decreased O2 concentration – This might be tempting, but we know that O2 and CO2 have an inverse relationship in the blood. CO2 is acidic, so any answer choice that increases the amount of CO2 in the blood is moving the blood pH in the acidic direction. That’s the opposite of what we’re looking for, as the blood is more basic or alkaline in this question.

(B) Decreased tidal volume – Tidal volume is the natural volume of air being exhaled per breath without forcibly trying to pull in or exhale more air. So if you’re exhaling less air, there is decreased tidal volume. This means that the CO2 is building up in your body since not as much is getting exhaled.

Remember, for the purposes of blood pH, CO2 is acidic. 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.

Remember, for the purposes of blood pH, carbon dioxide is acidic.Click To Tweet

(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 – Let’s say 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 your blood will become more basic and your blood pH could be higher. So this answer choice sounds more correct.

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.

So we choose answer D.

Question #17: Patients with excess fat are more likely to require larger therapeutic doses of which vitamin?

Answer choices:

  • (A) Vitamin B1
  • (B) Vitamin C
  • (C) Vitamin D
  • (D) Vitamin B3

Strategies for answering:

This question is a classic example of “Which one of these doesn’t belong?” You could almost answer it without looking at the question.

Looking at all the choices, one thing jumps out right away. You have to know your water-soluble and your fat-soluble vitamins to see it.

Vitamins B and C are water-soluble. Vitamins A, D, E and K are fat-soluble.

Hence, answer (C) Vitamin D is the only one that stands out.

Vitamins B and C are water-soluble. Vitamins A, D, E and K are fat-soluble.Click To Tweet

Question #29: Which of the following is most likely to use a protein channel to cross the eukaryotic cell membrane?

Answer choices:

  • (A) Aldosterone
  • (B) Ca2+ ion
  • (C) Oxygen (O2)
  • (D) Carbon dioxide

Strategies for answering:

This is just like the question about vitamin solubility mentioned before: Which of the answer choices doesn’t belong?

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 do need a protein channel. Hence, answer (B), the calcium ion, needs a protein channel to cross the membrane.

Steroid hormones don't need a protein channel. Gases don't need a protein channel. Ions do need a protein channel.Click To Tweet

If you’re looking for more discussion of discrete MCAT biology questions, check out this episode!

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