TLIK: You wrote “A real gadol does NOT give advice regarding subjects he does not know. It is the ignorant public that pushes the Rav to advise on medical subjects. But their advice there is actually lay knowledge, not Daas Torah. ”
The Chazon Ish was known for his expertise in medical situations. He did not have any “lay” knowledge. It is well known that a pediatrician in Eretz Yisrael kept the diagram for an operation that was drawn by the Chazon Ish. In addition, there was a medical conference in EY in the late Forties or Early Fifties. On of the speakers decried how the frum world went to the “black coat rabbis”. in fact one of his patients went to the Chazon Ish and followed what the Chazon Ish told him. Another doctor in the audience rose and said that the Chazon Ish conferred with him. The first question asked was how often is this doctor’s diagnosis incorrect? When told 25%, the Chazon Ish gave his advice.
Rav Pam was once asked advice by someone who had to undergo surgery for cancer. He went for two opinions. Both agreed that he needed surgery. Dr “A” was known as a very good surgeon; however, the oncologist suggested that he use Dr. “B”. He asked Rav Pam who advised to use Dr. “B” since the oncologist sees the patients after surgery he knows who the best surgeon is.
Rav Yaakov Kaminetsky was an exception. When he became rav in small Russian town, he realized there was no doctor. He stayed up all night reading a medical book. A few days later somebody in town took sick. Reb Yaakov called the doctor in the next town to discuss the situation. The other doctor was convinced that he was talking to a trained physician and not the Rav of the town.