Juries have the authority to acquite any defendant for any reason. Historically, juries have been made aware of this authority but--being responsible members of the community--juries would generally only acquit defendants for good reasons (the most common, of course, being that they didn't think the defendant committed the crime, but another common one being that the jury did not feel that a conviction would serve the interests of justice).
For example, if the police arrested someone who had just shot a homicidal maniac and arrested him for discharging a firearm in a public park, it would be right and proper for a jury to acquit the defendant no matter how clear and compelling the evidence that he did, in fact, discharge a firearm in a public park.
Although the jury system has at times resulted in some abuses (e.g. in some areas, whites who killed blacks would get acquitted even when proof of guilt was obvious) it served as a check against abusive laws. One of the reasons the right to jury is guaranteed is that it protected many colonists from oppression by the British throne. If a tax was widely unpopular, people might be arrested for evading it but then acquitted by a jury full of people who despised it just as much as the defendant.
Unfortunately, jurors are almost never informed of their right and responsibility to ensure that a conviction would be in the interest of justice. If they were, issues like this "mother's health" nonsense would be a non-factor because juries would naturally refuse to convict doctors who could convince them that their actions were medically necessary.
Jury nullification has nothing to do with my premise which is that juries and judges do not make law, legislative bodies do. A jury can make no law, though they can in indidual court cases nullify the law. I understand that.