Logical reasoning PrepTest 158 · Section 4 · Question 11

Argument or Facts:
Facts

Question Type:
Must Be True

Passage Summary:
Infants who were not breast fed were once fed cow's milk. Doctors began recommending boiling cow's milk given to infants. They said boiling cow's milk would sterilize the milk and prevent potentially fatal infections. After people implemented this recommendation, cases of scurvy in infants increased. This scurvy was caused by vitamin C deficiency. Breast-fed infants did not get scurvy.

Strategy Overview:

1. Argument or facts in passage?
1. If argument, identify premises and conclusions
2. Sufficient/necessary conditions in the passage?
1. If so, diagram statements and draw any deductions
3. Use anticipation to find the answer choice that is absolutely 100% true

Answer Anticipation:
Many Must Be True questions feature "if-then" statements, allowing us to connect these statements to draw a conclusion. However, this passage doesn't feature any "if-then" statements. Instead, we'll summarize the information and see if we can connect any of the facts.

Fortunately, the passage proceeds straightforwardly. Back in the day, kids were fed cow's milk or breast milk. Doctors made a recommendation to boil cow's milk before feeding it to infants. After this recommendation became widely implemented, a bunch of kids got scurvy. The scurvy was caused by vitamin C deficiency. The kids who drank breast milk didn't get scurvy.

From this information, we can deduce a few facts. First, the kids who got scurvy were fed boiled cow's milk. The first sentence tells us that cow's milk and breast milk were the only options available in these olden times, and the kids fed breast milk didn't get scurvy. And we know the kids got scurvy after the doctors' recommendation to boil cow's milk became widely implemented. So, these infants were fed boiled cow's milk.

We can also infer that the kids who consumed boiled cow's milk must have consumed less vitamin C than the kids drinking breast milk. After all, we know that the scurvy was caused by vitamin C deficiency. If the kids who consumed breast milk didn't get scurvy, they were not vitamin C deficient. So, the breast milk must have provided more vitamin C to these tots. 

Finally, the boiling probably reduced the vitamin C in cow's milk. There was an "alarming increase" of scurvy once people started boiling cow's milk. Unboiled cow's milk must provide some vitamin C if the rate of scurvy in infants was much lower when infants consumed unboiled cow's milk.

But we should be careful not to make any unsupported inferences. For instance, does breast milk have more vitamin C than unboiled cow's milk? We have no idea. Perhaps we could make this inference if the passage told us that fewer kids who drank breast milk contracted scurvy than those who consumed unboiled cow's milk. But, alas, we don't have that information.

So, let's focus on the inferences we can make (the kids who got scurvy were fed boiled cow's milk, boiled cow's milk has less vitamin C than breast milk, and the boiling process probably reduces vitamin C in cow's milk) and look for one of those in the answer choices.

Answer Choice Explanation:
This inference must be true! As we anticipated, boiled cow's milk must contain less vitamin C (or, at least, provide less vitamin C to infants, as this answer choice states) than breast milk (AKA mother's milk). We know that the infants who got scurvy were fed boiled cow's milk. And we know that the scurvy was caused by vitamin C deficiency. If the kids who consumed breast milk didn't get scurvy, they were not vitamin C deficient. So, the breast milk must have provided more vitamin C to these tots. That would mean the kids who drank boiled cow's milk did not consume as much vitamin C.

Because we anticipated this inference and can point to its support in the passage, we can justifiably select (A) and moooove on.

Key Takeaway:
Comparisons help us make inferences. In this case, knowing that there was an "increase" in the incidence of scurvy and that no breast-fed infants contracted scurvy helped us make the inference the correct answer describes.

This inference is tempting because we know that doctors advised boiling because it would "prevent gastrointestinal infections potentially fatal to infants." Many test-takers believe that it must be true that infants who consume unboiled cow's milk would frequently contract these gastrointestinal infections.

However, this inference is not supported by the text. First, we don't know that the doctors were correct that boiling cow's milk reduces the risk of gastrointestinal infections. Perhaps if we knew that there was a dramatic decrease in gastrointestinal infections among infants once most people began boiling their cow's milk, we could infer that the doctors' recommendation was sound. But the passage doesn't provide that information. Moreover, even if people weren't boiling their cow's milk before the doctors made their recommendation, maybe they did something else that reduced the risk of gastrointestinal infection. Finally, even if infants who consumed unboiled cow's milk were at risk of gastrointestinal infection, we don't know that they "frequently" contracted these infections. Perhaps these infections were very rare.

This inference is not supported. According to doctors, unboiled cow's milk can cause gastrointestinal infections in infants. But we weren't told whether breast milk (AKA mother's milk) poses the same risk. We also don't have any data on gastrointestinal infections among infants fed breast milk, so we can't infer that this claim is true.

This inference is not supported. Presumably, if the doctors knew that boiling cow's milk reduces the vitamin C levels in cow's milk and that scurvy is caused by vitamin C deficiency, they probably wouldn't have recommended boiling cow's milk. (Even then, they may have made the recommendation. They could have figured that the risk of gastrointestinal disease was more dangerous than scurvy. But let's ignore that possibility for now.) Since they made the recommendation, they either did not know that boiling cow's milk reduces its vitamin C levels or they did not know that scurvy is caused by vitamin C deficiency. So, it's entirely possible that the doctors knew that scurvy is caused by vitamin C deficiency, but they didn't know that boiling cow's milk reduces its vitamin C levels.

This inference isn't supported. We have no idea what proportion of mothers breast fed their infants. We could make this inference if we learned that "most infants" contracted scurvy, but breast-fed infants did not contract scurvy. However, the passage does not supply that information.