Q: Our 5-year-old boy is 3 feet, 9 inches tall and weighs 37 pounds. He's basically healthy and eats small portions of a well balanced diet. He seems to naturally shun high-calorie foods, except for certain sweet treats. My concern is that he's beginning to become self-conscious when anyone mentions his thinness. Should I try to fatten him up, or is his body type basically predetermined?

A: Your son is somewhat taller than average for his age, and lighter in weight. Put another way, he's thin for his height.

Doctors use growth curves to keep track of children's growth, plotting height and weight measurements at different ages. By watching the progress of growth over time, you can spot trends, judge when children are growing normally and tell when something is going wrong with their physical development.

Assuming your son has been growing nicely, following his individual growth curve, I don't see any cause for concern. He's probably meant to be on the thin side, as you suspect. (Another way to judge this is by taking into account the height, weight and body builds of a child's parents, which he tends to mirror.)

The question, then, is what to do about his self-consciousness about being thin. This can be difficult for anyone, as children have a way of finding fault with each other (being thin, heavy, wearing glasses, having a stutter and so forth).

I suggest that you keep letting him know you think he's fine and lovable. It may also help if he has some role models (relatives or sports figures he admires) with similar builds.

Q: I'm a 26-year-old woman and have repeatedly had all the symptoms of a bladder infection over the past few months -- burning with urination, going to the bathroom frequently, constantly feeling the urge to void, and itching. My urine culture never shows an infection, so the doctor doesn't seem to know what to do with me. Do you have any suggestions?

A: I have a few ideas about what might be causing your problem, but first it's important to find out a few things.

If your urine has many white blood cells in it, there's a good chance you do have a bladder infection, but with an organism that doesn't show up on the usual urine cultures. One of these is called chlamydia, and it's probably the most common cause of symptoms of a bladder infection with a normal urine culture in a young woman. (This situation is different for women after menopause.) Other organisms that can do this are gonorrhea, herpes, trichomonas and yeast infections (the first two are unlikely because of the duration of your symptoms). Also, the urine culture may have actually shown something, but not in what doctors call "significant" amounts. Given your symptoms, it's probably best to treat any organism that is found, even in less than significant amounts.

Next, it's important to make sure your symptoms aren't actually due to a vaginal infection. The best clue to this is whether the burning with urination seems to feel internal or external. Internal burning is likely to be caused by a bladder infection, external burning by a vaginal infection. If you haven't had a pelvic examination to check for vaginal infection, I'd recommend one. In fact, your itching is not a typical symptom of a bladder infection, and it makes me wonder whether you might have a vaginal yeast infection instead.

It's also possible that something besides an infection is causing your symptoms, including irritation from ingredients in bubble baths, feminine hygiene sprays and contraceptive foams or jellies.

Finally, if none of these possibilities proves to be the case, I'd suggest seeing a urologist to have your bladder examined for something called interstitial cystitis -- bladder inflammation without any infection. This condition is much less common than bladder infection but more difficult to treat.

Q: I have an annoying and embarrassing problem. Any time I go outside when the weather is brisk and I walk or bike ride (any type of exercise, really), my lower body begins to itch and turn red. As soon as I get inside, the itching and red rash disappear. I don't have any allergies or take any medicines. I've never heard of anything like this. What do you think is going on?

A: It sounds like you might have something called cholinergic urticaria. Urticaria is the medical term for hives, and cholinergic refers to acetylcholine, a substance that plays a role in the way nerves work. The hives of cholinergic urticaria are distinctive tiny red bumps, unlike the large red blotches more typical of hives.

Although hives are often an allergic reaction, cholinergic urticaria is an uncommon type of hives brought on by nonallergic factors -- heat (hot baths, exercise, sun exposure, fever) or excitement. In your case, exercise is probably triggering the release of acetylcholine to produce the skin reaction you're having.

Unlike some other forms of urticaria which may be difficult to diagnose or treat, cholinergic urticaria usually responds to 1) avoidance of the triggering factors or 2) antihistamine medications such as hydroxyzine (Atarax, Vistaril).

My guess is that it's the exercise and not the brisk weather that sets off your skin reaction. But just in case it's the other way around, I'll mention that there is another type of urticaria called cold urticaria, which is triggered by cold temperatures. Cyproheptadine (Periactin) usually helps this form of hives.