PSAT Reading Question 503: Answer and Explanation

Question: 503

According to the third paragraph (lines 17–31), the presence of methanogenic species in the human digestive system was first detected from

• A. observation of a product of a chemical reaction.
• B. isolation from human bowel movements.
• C. the decoding of the DNA of methanogens.
• D. the discovery of its essential role in human homeostasis.

Correct Answer: A

Explanation:

(A) According to lines 17–20, methanogenic bacteria were first identified in the gut microbiota after methane was detected in the breath. Methanogens are defined throughout the passage as species that produce methane, so this presence of methane in the breath eventually led to the identification of methanogenic species in the human digestive system. Thus, the answer is choice (A). Choice (B) is incorrect because it says that the methanogens were eventually identified in fecal samples, but this wasn't how they were originally discovered. Choices (C) and (D) are details of the passage mentioned in other paragraphs but not in the third paragraph and not about how methanogens were discovered.