<p>Today a medical doctor must often make a choice between engaging in private practice and engaging in research.</p>
<p>Answer: No Error
My Question: How is "make" in the right number when it refers back to the singular "medical doctor"?</p>
<p>The verb “must” functions here as a modal auxillary verb. (Since the verb “must” is not conjugated, it agrees with any and all subjects.)</p>
<p>See [Learn</a> English online - English Grammar - Modal auxiliary verbs](<a href=“http://www.perfectyourenglish.com/grammar/modal-auxiliaries.htm]Learn”>http://www.perfectyourenglish.com/grammar/modal-auxiliaries.htm).</p>