BS #17 on FL2

Is there a faster way to think thru essential amino acid questions for microbes?
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This question asks us to identify an essential amino acid for microbe Q.  If it's essential, that means that it can't survive without being given this amino acid from the medium; in other words, there will be no growth if the plate doesn't contain that amino acid.  This is one of the few questions that it's probably better to go to the answers than to predict.  The possibilities in the answers are:  (A) serine; (B) threonine; (C) phenylalanine; (D) proline.  Looking at the plates and the chart, we see that there is growth on plates 1 and 3, but not 2 or 4.

Plate 1 (GROWTH): cysteine, phenylalanine, serine, threonine

Plate 2 (NO GROWTH): cysteine, phenylalanine, proline, tryptophan

Plate 3 (GROWTH): cysteine, proline, threonine, tryptophan

Plate 4 (NO GROWTH): phenylalanine, serine, threonine, tryptophan

Let's start with serine.  Serine can't be essential because we have growth on plate 3, without serine.
Next, threonine.  This could be essential because we never have growth without it.
Phenylalanine can't be essential because plate 3 has growth without it.
Proline, similarly, can't be essential because we have growth on plate 1 without it.

Another way of looking at it is to ask what amino acid plates 1 and 3 share.  Since there is growth on both plates, they must have the essential amino acid.  The only two they share are cysteine and threonine.  Threonine is an answer; hence, that's the right answer.