MCAT Biology Question 137: Answer and Explanation
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Question: 137
2. In which of the following segments of the nephron is sodium NOT actively transported out of the nephron?
- A. Proximal convoluted tubule
- B. Thin portion of the ascending limb of the loop of Henle
- C. Distal convoluted tubule
- D. Thick portion of the ascending limb of the loop of Henle
Correct Answer: B
Explanation:
Sodium is actively transported out of the nephron in the proximal and distal convoluted tubules, where the concentration of sodium outside of the nephron is higher than inside; thus, energy is required to transport the sodium molecules against their concentration gradient, eliminating choices (A) and (C). In the inner medulla, however, sodium and other ions (such as chloride) diffuse passively down their concentration gradients from the thin ascending limb of the loop of Henle, making choice (B) the correct answer. The thick ascending limb of the loop of Henle is thick because its cells contain many mitochondria—which produce the ATP needed for active transport of sodium and chloride out of the filtrate, eliminating choice (D).