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The best initial test for evaluating acromegaly is measuring insulin-like growth factor 1 (IGF-1) levels in the serum. In acromegaly, which is often caused by a growth hormone-secreting pituitary adenoma, IGF-1 is typically elevated. This makes IGF-1 a useful screening tool because it reflects average growth hormone secretion over time, overcoming the variability of growth hormone levels due to its pulsatile nature.

When considering the other options, serum acid phosphatase is not relevant for diagnosing acromegaly and is more associated with conditions like prostate cancer. The growth hormone suppression test, which is used to confirm acromegaly after initial screening, involves measuring growth hormone response to an oral glucose tolerance test; however, this is not the first step in evaluating the condition. Lastly, imaging studies like a CT scan of the pituitary would generally come after an abnormal screening result to assess for a tumor, reinforcing the rationale behind starting with an IGF-1 measurement.