DEAR DR. DONOHUE: Please explain ringworm to me. I have a rather large one at the top of my leg. My doctor prescribed ketoconazole cream. I’ve used it for longer than a week, but it’s no better. My girlfriend’s daughter just graduated from medical school. She told her mother that there’s a worm in the center. Surely this isn’t true, is it? What causes ringworm? I am 81 years old. — V.C.

ANSWER: Ringworm got that name because the circular patch of red skin is surrounded by a slightly raised border that looks like a worm. It never looked like a worm to me, but it must have to the guy who gave it this name. The medical name is tinea, a Latin word for “worm.” It’s a fungal infection, and more common in younger than older people.

The infection can be in many different places: on the head, it’s called tinea capitis; on the body, tinea corporis; on the feet, tinea pedis, or athlete’s foot. People catch ringworm fungi in many ways. Usually it’s from infected skin scales that have fallen off a person with ringworm. Shower-room floors, locker rooms and similar places are where most infections are caught. Wrestlers get it from skin-to-skin contact. Sometimes animals are involved, but not too often.

An infection is mostly a visual thing. Most people have no symptoms. When they do, itching is the biggest complaint.

Thirteen creams, gels or lotions are available to treat ringworm. Yours is one of them. They’re usually effective. If they aren’t getting the job done, oral medicines can finish the fungus off.

You’re at an age when ringworm infections are not common. You have not been in situations or places where transfer is probable. I’m skeptical about the diagnosis. Next time you see the doctor, poke him or her in the chest and say: “Look, doc, I want some proof. Take some skin scrapings and let me see the fungus with your microscope or send those scrapings to the lab for culture.” And as an aside, please don’t tell me the name of the medical school your friend’s daughter graduated from.

DEAR DR. DONOHUE: I am 75 years old. I can’t eat Splenda, diet candy, diet gelatin or diet anything. I can eat only Equal. The others give me diarrhea. What’s wrong with me? — J.O.

ANSWER: Nothing’s wrong with you. Your body might have an intolerance to those other artificial sweeteners. Humans are not carbon copies of each other.

In large amounts, artificial sweeteners can bring on gas, bloating and diarrhea. Alcohol sugars, which are neither alcohols nor sugars, are nonsugar sweeteners that, in excess, cause diarrhea. They’re found in sugarless gums, candies and other products.

If you tolerate Equal, you should be able to tolerate NutraSweet. Both are the same artificial sweetener aspartame.

DEAR DR. DONOHUE: Last week, while looking in a mirror, I was shocked by the sight of my tongue. It was covered with red patches having white borders.

I called my dentist and asked to be seen right away. The receptionist gave me a hard time, but she grudgingly got me in.

The dentist was just as abrupt with me. He dashed in, and I showed him my tongue. He said it was geographic tongue, and it wasn’t an emergency, and there was nothing to do for it. Then he dashed out. I felt like a fool. Can you give me a little more information? — C.N.

ANSWER: Geographic tongue is a tongue with red patches that have a white border. What’s really spooky is that the shape and distribution of the patches change. The tongue looks like a relief map. That’s where the “geographic” comes in.

Things straighten out in time. No treatment is needed. You might want to consider a change of dentists.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.