The Queen has been described as "very on the ball" after visiting the Royal College of Physicians (RCP) to mark its 500th anniversary.
It was the Queen's first public engagement outside of Buckingham Palace since she returned to London from Sandringham.
Dressed in a duck egg blue Stewart Parvin dress and matching coat, the 91-year-old Queen was greeted by RCP president Professor Jane Dacre, who was impressed by her royal visitor.
"Very on the ball, very interested, and a charming, graceful lady," Prof Dacre said of the Queen after the event.
In a speech, Prof Dacre - the RCP's third female president - joked: "I hope I am not being presumptuous, Ma'am, in thinking that Ma'am will be most relieved that medical treatment has changed.
"It's moved on from when monarchs were treated with arsenic and bloodletting."
After the Queen left, Prof Dacre said: "She really seemed genuinely interested."
During her visit to the College's Regent's Park headquarters in central London, the Queen viewed an exhibition called Ceaseless Motion: William Harvey's Experiments In Circulation.
It celebrates the life and works of physician William Harvey whose discovery of the circulation of blood changed the face of medicine.
Kristin Hussey, curator at the RCP, said: "She was quite keen to see the bloodletting. And she said that she thought it was really fascinating."
Ms Hussey said the Queen and Prof Dacre had a conversation "about the importance of diversity in medicine".
Prof Dacre said afterwards that there have been more Queens than female presidents of the RCP.
Before the Queen arrived, Prof Dacre said: "One thing that's very poignant for me is that I'm only the third ever female president.
"And I'm very proud to be here on this occasion."
Asked about the NHS, Prof Dacre said: "I think the NHS is troubled.
"However, every health system in the world is troubled.
"And if you look at the Commonwealth Fund, it's an American piece of research that looks at how healthcare systems are doing, it puts us right at the top.
"So we are amongst the best health systems in the world.
"We're priceless to have healthcare for everybody free at the point of delivery.
"The difficulties with that is that we've become victims of our success, because people are living longer and collecting more illnesses and we're treating them all and so it means there's a lot of pressure.
"Having said that though, the job of being a doctor and caring for an individual patient in front of you is a huge privilege."
During the visit, the Queen also unveiled a plaque to mark the milestone and was shown the original charter document from 1518, granted by Henry VIII, which founded the College.
It was established to help regulate the medical profession at a time when anyone could call themselves a doctor and start treating patients.
Today the RCP plays an important role in improving patient care and shaping public health and is the professional membership body for physicians with 34,000 members and fellows across the globe.
The Queen met staff and members of the medical profession, before unveiling the plaque and a new commemorative charter to mark the occasion.
The College will be organising a series of events throughout 2018 to mark the 500th anniversary of the body's original 1518 royal charter.
It has also produced a commemorative RCP500 Charter, to reaffirm the College's commitment to seek and champion excellence in healthcare - through research, training and support for members of the medical profession.
The Queen laid the foundation stone for the College's Regent's Park building in 1964 and Tuesday's visit came at the start of a year which will also mark the 70th anniversary of the NHS.