Hi there! This is a great question. Let's break it down into two parts: first, is hGH tissue-specific or tissue-nonspecific? And second, is its expression directed by the
Nbea promoter, or by something else?
We want to examine these two parts completely independently. In other words, when deciding whether hGH expression is tissue-specific or nonspecific, don't think about
Nbea at all - just consider what "tissue-specific" means. When gene expression is tissue-specific, it simply means that it is expressed to a different extent in different types of tissue. (Often, in a lab/research context, this means that the gene is predominantly expressed in one kind of tissue. For example, a gene could be expressed only in nervous tissue. But as long as it's expressed differently in different tissue types, it's tissue-specific.)
From this alone, we KNOW that hGH must be tissue-specific, because the question states that it isn't expressed at all in the hippocampus. Since hGH certainly IS expressed in some cells/organs/tissues, its expression clearly differs depending on the tissue/type of cell. This eliminates choice B and D automatically.
Next, we need to attack the second part of the answer choices. The question stem tells us that the highest expression of
Nbea is found in the hippocampus, so if the
Nbea promoter also determined where hGH was expressed, we'd expect lots of hGH in the hippocampus. Since this clearly isn't the case, we can eliminate C, since
something else must determine where hGH is expressed. Even if you aren't sure about the whole "promoter elements in the hGH transgene" business, we know that choice A must be correct, since we've just eliminated choice C.
In summary, you're totally right that
Nbea doesn't seem to determine where hGH is expressed. Where your reasoning went wrong was confusing this with the concept of tissue specificity. Just remember "tissue-specific = expressed differently depending on the tissue type" and you'll be good to go.
Good luck studying
