CATSprint

CAT 2025Slot 2VARCQues & Sol

Reading ComprehensionHard

Passage

The passage below is accompanied by four questions. Based on the passage, choose the best answer for each question.

Time and again, whenever a population [of Mexican tetra fish] was swept into a cave and survived long enough for natural selection to have its way, the eyes disappeared. "But it's not that everything has been lost in cavefish . . . Many enhancements have also happened." . . . Studies have found that cave-dwelling fish can detect lower levels of amino acids than surface fish can. They also have more tastebuds and a higher density of sensitive cells alongside their bodies that let them sense water pressure and flow. . . .

Killing the processes that support the formation of the eye is quite literally what happens. Just like non-cave-dwelling members of the species, all cavefish embryos start making eyes. But after a few hours, cells in the developing eye start dying, until the entire structure has disappeared. [Developmental biologist Misty] Riddle thinks this apparent inefficiency may be unavoidable. "The early development of the brain and the eye are completely intertwined—they happen together," she says. That means the least disruptive way for eyelessness to evolve may be to start making an eye and then get rid of it. . . .

It's easy to see why cavefish would be at a disadvantage if they were to maintain expensive tissues they aren't using. Since relatively little lives or grows in their caves, the fish are likely surviving on a meager diet of mostly bat feces and organic waste that washes in during the rainy season. Researchers keeping cavefish in labs have discovered that, genetically, the creatures are exquisitely adapted to absorbing and storing nutrients. . . .

Fats can be toxic for tissues, [evolutionary physiologist Nicolas] Rohner explains, so they are stored in fat cells. "But when these cells get too big, they can burst, which is why we often see chronic inflammation in humans and other animals that have stored a lot of fat in their tissues." Yet a 2020 study by Rohner, Krishnan and their colleagues revealed that even very well-fed cavefish had fewer signs of inflammation in their fat tissues than surface fish do. Even in their sparse cave conditions, wild cavefish can sometimes get very fat, says Riddle. This is presumably because, whenever food ends up in the cave, the fish eat as much of it as possible, since there may be nothing else for a long time to come. Intriguingly, Riddle says, their fat is usually bright yellow, because of high levels of carotenoids, the substance in the carrots that your grandmother used to tell you were good for your…eyes.

"The first thing that came to our mind, of course, was that they were accumulating these because they don't have eyes," says Riddle. In this species, such ideas can be tested: Scientists can cross surface fish (with eyes) and cavefish (without eyes) and look at what their offspring are like. When that's done, Riddle says, researchers see no link between eye presence or size and the accumulation of carotenoids. Some eyeless cavefish had fat that was practically white, indicating lower carotenoid levels. Instead, Riddle thinks these carotenoids may be another adaptation to suppress inflammation, which might be important in the wild, as cavefish are likely overeating whenever food arrives.

Question 1

All of the following statements from the passage describe adaptation in Mexican tetra cavefish EXCEPT:

Solution

1. What the Question Asks

This is an EXCEPT question — you must identify the ONE statement that does not describe an adaptation in Mexican tetra cavefish. The trap here is that all four options are real quotes from the passage, so you must evaluate what each statement is actually doing in context, not just whether it appears in the passage.

2. Key Idea & Flow

The passage discusses a range of biological adaptations — from eye loss to nutrient storage to inflammation control — that help cavefish survive in resource-scarce caves. The key is distinguishing statements that describe a cavefish-specific adaptive trait from those that merely provide background biological context about other organisms. As Rohner explains, "But when these cells get too big, they can burst, which is why we often see chronic inflammation in humans and other animals that have stored a lot of fat in their tissues." — this statement is about humans and other animals, not cavefish, and is used as contrast to set up the remarkable finding that cavefish avoid this problem.

3. Elimination Approach

  • 🟡 Option op1: This statement describes why losing eyes is advantageous for cavefish — it directly points to the adaptive benefit of not maintaining energy-expensive unused tissue. It is clearly about a cavefish adaptation and must be eliminated.

  • 🟡 Option op2: This statement describes the environmental pressure cavefish face — a meager diet that has driven their metabolic adaptations. While it focuses on the environment rather than the fish's biology, it contextually supports and explains why cavefish adaptations evolved, making it part of the adaptation narrative.

  • 🟢 Option op3: This statement describes what happens in humans and other animals when fat cells burst — causing chronic inflammation. It serves as a contrast to highlight the cavefish's ability to avoid inflammation. It says nothing about a cavefish adaptation itself; it is background information about other species. This is the EXCEPT answer.

  • 🟡 Option op4: This describes cavefish becoming very fat in sparse cave conditions — a behavioral and metabolic adaptation to feast when food arrives. It directly reflects an adaptive survival strategy and must be eliminated.

4. Final Conclusion

Answer = Option op3 ✅

Option op3 is the only statement that describes a phenomenon in humans and other animals, not in cavefish — it provides biological background as a contrast, not an adaptive trait of the cavefish themselves.

Question 2

Which one of the following best explains why the "apparent inefficiency" is "unavoidable"?

Solution

1. What the Question Asks

The question asks you to identify the biological reason why cavefish embryos must begin forming an eye only to later destroy it — a process described as an "apparent inefficiency." The key challenge is understanding why this cannot be avoided, not just describing what happens.

2. Key Idea & Flow

The passage explains the developmental constraint directly: "The early development of the brain and the eye are completely intertwined — they happen together." Because brain and eye formation are linked in early embryonic development, removing eye formation entirely from the start would disrupt brain development. As a result, "the least disruptive way for eyelessness to evolve may be to start making an eye and then get rid of it." The inefficiency is unavoidable because cavefish embryos — like their surface-dwelling relatives — share the same early developmental blueprint, and decoupling eye formation from that blueprint is evolutionarily impossible without causing collateral damage.

3. Elimination Approach

  • 🔴 Option op1: This addresses why eyes are energetically costly in adult cavefish living on a meager diet. It explains the selective pressure to lose eyes, but does not explain why the embryo must still begin forming eyes in the first place. Irrelevant to the developmental constraint being described.

  • 🔴 Option op2: This incorrectly attributes eye cell death to darkness, implying an environmental cause. The passage is explicit that the loss is driven by developmental and evolutionary constraints, not by the absence of light directly triggering cell death in embryos.

  • 🔴 Option op3: This describes compensatory enhancements (more tastebuds, better pressure sensing) that follow eye loss. These occur after the inefficiency has already happened — they explain how cavefish cope, not why the inefficiency is unavoidable in the first place.

  • 🟢 Option op4: This correctly captures the passage's explanation — because cavefish embryos develop identically to surface fish in early stages, and brain and eye formation are intertwined in that shared blueprint, evolution cannot simply skip the eye-formation step without disrupting brain development. The shared early development is exactly why the inefficiency is unavoidable.

4. Final Conclusion

Answer = Option op4 ✅

Since cavefish embryos follow the same early developmental program as surface fish — where brain and eye formation are inseparably linked — evolution is forced to begin eye formation and then dismantle it, making the inefficiency biologically unavoidable.

Question 3

Which one of the following results for the cross between surface fish (with eyes) and cavefish (without eyes) would invalidate Riddle's inference from the experiment?

Solution

1. What the Question Asks

This is an invalidation question — you must find the result that would disprove Riddle's inference. First, you need to clearly understand what Riddle's inference is, and then identify which result creates a direct logical contradiction. The trap is that some options may sound unusual or surprising without actually undermining Riddle's conclusion.

2. Key Idea & Flow

Riddle's inference has two parts: (1) carotenoid accumulation (yellow fat) is not linked to eye loss, and (2) carotenoids likely serve as an adaptation to suppress inflammation. The experiment supports this because "researchers see no link between eye presence or size and the accumulation of carotenoids. Some eyeless cavefish had fat that was practically white." To invalidate this, you would need a result that establishes a clear, exclusive link between eyelessness and yellow fat — proving that only eyeless fish accumulate carotenoids, which would restore the original hypothesis that carotenoids accumulate because eyes are absent.

3. Elimination Approach

  • 🟡 Option op1: Some offspring with eyes had white fat. This shows that having eyes does not guarantee yellow fat — which actually supports Riddle's claim that eye presence and carotenoid accumulation are uncorrelated. This does not invalidate her inference.

  • 🟡 Option op2: Some offspring with eyes had yellow fat. This directly supports Riddle's argument — fish with eyes can also accumulate carotenoids, confirming the trait is independent of eye loss. This strengthens, not weakens, her inference.

  • 🟢 Option op3: Only eyeless offspring had yellow fat. This would mean every fish with yellow fat was eyeless and every fish with eyes lacked yellow fat — establishing a perfect, exclusive correlation between eye loss and carotenoid accumulation. This directly contradicts Riddle's conclusion by reviving the initial hypothesis she rejected.

  • 🔴 Option op4: Some eyeless offspring had white fat. The passage itself already reports this exact finding: "Some eyeless cavefish had fat that was practically white." This result is already part of the evidence that supports Riddle's inference, not a challenge to it.

4. Final Conclusion

Answer = Option op3 ✅

If only eyeless offspring had yellow fat, it would establish an exclusive causal link between eye loss and carotenoid accumulation — directly contradicting Riddle's finding that there is "no link between eye presence or size and the accumulation of carotenoids."

Question 4

On the basis of the information in the passage, what is the most likely function of carotenoids in Mexican tetra cavefish?

Solution

1. What the Question Asks

The question asks for the most likely function of carotenoids in Mexican tetra cavefish, based on the passage. The phrase "most likely" is significant — you should anchor your answer in the passage's explicit conclusions, not in general knowledge about carotenoids. The trap is the mention of carotenoids being linked to eye health (carrots and eyes), which is a red herring deliberately introduced by the author.

2. Key Idea & Flow

The passage walks through two hypotheses about carotenoids. First, researchers speculated that cavefish accumulated carotenoids because they lack eyes. However, the cross-breeding experiment disproved this. The final, authoritative conclusion in the passage is Riddle's: "Instead, Riddle thinks these carotenoids may be another adaptation to suppress inflammation, which might be important in the wild, as cavefish are likely overeating whenever food arrives." This connects directly to the earlier discussion about fat cells bursting and causing chronic inflammation — a problem cavefish seem to avoid. Carotenoids, then, are most likely serving an anti-inflammatory role.

3. Elimination Approach

  • 🔴 Option op1: This reflects the initial, discarded hypothesis — that carotenoids accumulate to substitute for lost eyes. The passage explicitly rejects this: "researchers see no link between eye presence or size and the accumulation of carotenoids." This is the author's planted red herring using the grandmother's comment about carrots and eyes.

  • 🔴 Option op2: The passage never links carotenoids to nutrient storage in fat cells. Fat storage is discussed as a separate metabolic adaptation. Carotenoids are mentioned only in the context of inflammation, not as a mechanism for storing nutrients.

  • 🔴 Option op3: While the passage does note that carotenoids make the fat "bright yellow," this is merely a physical observation, not a function. The yellow colour is a visible symptom of carotenoid presence, not the biological purpose for which they are accumulated.

  • 🟢 Option op4: This aligns precisely with Riddle's explicit conclusion. The passage establishes that overeating causes fat cells to enlarge, which can burst and cause inflammation. Cavefish appear to suppress this inflammation — and Riddle attributes this to carotenoids acting as an anti-inflammatory adaptation.

4. Final Conclusion

Answer = Option op4 ✅

Riddle's explicit conclusion — that carotenoids are "another adaptation to suppress inflammation" in cavefish that overeat whenever food arrives — directly points to controlling inflammation from fat cell bursting as the most likely function.