Among the health problems that necessitate liver transplants, the vir

For which is correct usage in this sentence and affects is also with singular subject.

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Kaplan official solution
Read the Original Sentence Carefully, Looking for Errors:

This underlined portion contains a long clause that separates the subject of the sentence ("the ... disease hepatitis C") and its verb ("affect"). The GMAT commonly uses long intervening phrases to mask errors with subject-verb agreement. Here, the subject is singular while the verb is plural, so this is an error. In addition, the underlined portion begins with "in which," words that simply do not convey the logical intended meaning. The noninvasive therapy is not working in the virus. The proper idiom is for which.

Scan and Group the Answer Choices:

Using the verbs that end each choice, the answer choices can be grouped in a 2-2-1 split. (A) and (C) use the plural verb “affect,” while (B) and (D) use the singular verb “affects.” (E) changes the verb “affect” to the gerund “affecting.”

The choices can also be grouped into a 2-2-1 split on the basis of the beginning of each choice. (A) and (B) begin with “in,” while (C) and (D) begin with “for.” (E) eliminates the phrase entirely.

Eliminate Wrong Answer Choices:

Since the disease is singular while the verb “affect” is plural, you can quickly eliminate (A) and (C).

(E) incorrectly changes the verb “affect” to the gerund “affecting.” This choice creates a sentence fragment that lacks a verb, so it should be eliminated.

(B) and (D) both use the singular verb “affects,” but only (D) uses the proper preposition “for.”(D) is the correct answer.

If you were unable to spot the subject-verb agreement error, you could also eliminate choices by looking at the first word of each choice. (A) and (B) use the wrong preposition for the context. A therapy works for a disease, not in a disease. (E) removes the “for” at the beginning of the underlined portion and changes the sentence to mean that the “viral disease hepatitis C” is the noninvasive therapy. When you compare the two remaining choices, (C) and (D), if you don’t spot the difference in the verbs, you may notice that (C) incorrectly uses the idiom “affect beyond” while (D) uses the proper idiom “affects over.”

TAKEAWAY: Even if you don't spot every error in a sentence, you can often get to the correct answer using what you have identified.

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