There are two ways to approach this issue.

1. The grammar way: We know as per the V-A-N rule, a verb prevails over adjective and a noun. Here the phrase 'synthetic' is an adjectival and is inferior 'to synthesize the hormone growth' which is a verb. Therefore, A, B, and C are out.
Between D and E, E has a severe modification error in that, the noun Scientists should have been placed immediately after the comma. D is the correct choice.

2. The meaning way: It may be noted that a synthetic hormone is made only outside the body and not within the body. Choices A, B and C all erroneously point to the removal of the synthetic growth hormone from the pituitary glands, which are inside the living body; therefore, the first three choices are eliminated.
Now it is the choice between D and E, wherein as such the pronoun 'it' pointedly refers to the growth hormone. However, D is better than E because of the modification flaw in E.

daagh I just have one confusion I understand why each option is wrong except D what I don’t understand is that since the timeline is clear as we have used “before” in the stmt doesn’t that make the use of “had” redundant or wrong?

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Before learning how to make a synthetic growth hormone, scientists had to remove it painstakingly in small amounts from the pituitary glands of human cadavers.

(A) learning how to make a synthetic growth hormone, scientists had to remove it painstakingly

(B) scientists learned how to make a synthetic growth hormone, they removed it painstakingly

(C) scientists learned how to make a synthetic growth hormone, they had to remove it painstakingly

(D) scientists learned how to synthesize the growth hormone, it had to be painstakingly removed

(E) learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists

Before learning how to make a synthetic growth hormone, scientists had to remove it painstakingly in small amounts from the pituitary glands of human cadavers.

(A) learning how to make a synthetic growth hormone, scientists had to remove it painstakingly

(B) scientists learned how to make a synthetic growth hormone, they removed it painstakingly

(C) scientists learned how to make a synthetic growth hormone, they had to remove it painstakingly

(D) scientists learned how to synthesize the growth hormone, it had to be painstakingly removed

(E) learning how to synthesize the growth hormone, it had to be painstakingly removed by scientists

make a synthetic

scientist

First, bear in mind that a pronoun functions as a stand-in for the entire noun phrase it refers to. For example:

Because Tim had heard an urban legend that Taco Bell tacos are made with horse meat, he decided to make them at home instead, using his own recipe, which called for hamster meat.

Here "them" refers to "Taco Bell tacos." But this doesn't make sense. Tim isn't making Taco Bell tacos - he's making his own. Put another way, if we include the modifier "Taco Bell," when we consider what the pronoun is standing in for, we get an illogical meaning.

We see the same problem here with (A), (B), and (C). As others have noted, for these three answer choices, the "it" in the second clause seems to refer back to "a synthetic growth hormone." If we substitute "a synthetic growth hormone" into the second clause, we get some variation of the following:

"...remove a synthetic growth hormone painstakingly in small amounts from the pituitary glands of human cadavers..."

Synthetic growth hormone is, by definition, synthesized in a lab. What on earth is it doing in human cadavers? Were the scientists performing twisted experiments on these cadavers, injecting them with lab-made hormones to see what would happen? Alas, contrary to popular belief, H.P. Lovecraft was not a GMAT question-writer, so (A), (B), and (C) are out.

Next we get some seriously low-hanging fruit in (E): "learning how to synthesize the growth hormone, it..." "It" is learning how to synthesize? What could "it" even refer to here? The growth hormone? Clearly, it should be the scientists doing the learning. So (E) is out and (D) is the only one left standing.

I hope that helps!

Hi GMATNinja Can you please explain what 'it' is referring to in Option D. To me, It seems it is also referring to Growth hormone.

Thanks Much

Isn't Option D is incorrect since two independent clauses are connected by comma only?

Dear junii
Option D consist of dependent and independent clauses. "Before" is a subordinate conjunction that links dependent [scientists learned...] and independent clauses [it had to be...]. Hence, comma is utterly legitimate here.

Before scientists learned how to synthesize the growth hormone, it had to be painstakingly removed

Dear BLTN Thank you for your response. I don't get how first part is dependent clause? Scientists is subject learned main verb isn't it?

Scientists is subject learned main verb isn't it?