MCAT Biology Question 87: Answer and Explanation

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Question: 87

12. A patient presents to the emergency room with an asthma attack. The patient has been hyperventilating for the past hour and has a blood pH of 7.52. The patient is given treatment and does not appear to respond, but a subsequent blood pH reading is 7.41. Why might this normal blood pH NOT be a reassuring sign?

  • A. The patient's kidneys may have compensated for the alkalemia.
  • B. The normal blood pH reading is likely inaccurate.
  • C. The patient may be descending into respiratory failure.
  • D. The patient's blood should ideally become acidemic for some time to compensate for the alkalemia.

Correct Answer: C

Explanation:

When a patient with an asthma attack does not respond to treatment and has been hyperventilating for over an hour, he or she may become fatigued and may not be able to maintain hyperventilation. In this case, the patient begins to decrease his or her breathing rate and is not receiving adequate oxygen. By extension, carbon dioxide is trapped in the blood, and the pH begins to drop. Despite the fact that this pH is normal at the moment, this patient is crashing and may start demonstrating acidemia in the near future. While the kidneys should compensate for alkalemia, this is a slow process and would not normalize the blood pH within an hour; further, adequate compensation by the kidneys would actually be a reassuring sign, eliminating choice (A). There is no evidence to believe the measurement was inaccurate, eliminating choice (B). Finally, after treatment, the patient should return to a normal blood pH with adequate ventilation and would not be expected to overcompensate by becoming acidemic, eliminating choice (D).

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