Q: I read your answer about the tall, thin boy, but my question is a little different. My 4-year-old granddaughter is not growing and is very small. Although her parents are both short, she weighs only 23 pounds and is wearing last year's clothes. She seems to be in good health and is bright. She was seen by a doctor who didn't find anything the matter. Are there any tests to tell whether she is just naturally very small or whether something is internally wrong, as I suspect?

A: I share your concern.

A 4-year-old girl should weigh somewhere between 30 and 50 pounds. At 23 pounds, she falls into the category of what doctors call "failure to thrive."

Some causes of this syndrome are:

*Naturally small size -- a hereditary condition.

*Poor nutrition.

*Lack of appropriate emotional contact with a parent or parent substitute.

Diseases such as underactive thyroid (hypothyroidism), cystic fibrosis, or inability to absorb or properly use necessary nutrients, which occurs in many chronic illnesses.

To find the cause of failure to thrive, doctors start with a thorough history and physical examination. If no clues are uncovered, children are sometimes admitted to a hospital, fed and observed for weight gain. If they gain weight in the hospital, doctors look for problems in proper nutrition at home, family conflicts or difficulties in parenting.

One way to judge whether there may be an internal problem is to take an X-ray of the wrist. Doctors can estimate a child's age by the appearance of the bones and compare this with the known age. If there is a big difference, something is amiss.

Some common tests used to screen for internal disorders are a complete blood count, urine tests and a measurement of the thyroid hormone.

The more clues that point to something other than just natural smallness, the more your doctor will want to pin down the cause.

Q: I occasionally get bursts of rapid heartbeat known as paroxysmal atrial tachycardia (PAT). I also have a heart condition called Wolff-Parkinson-White syndrome (WPW). Only recently, I found out that the two are related and would like to know how. I have also been told by several doctors not to take over-the-counter medicines like decongestants because they might aggravate my condition. What other medicines should I avoid and why?

A: Named after the three doctors who discovered it, WPW syndrome refers to an abnormal arrangement of nerves within the heart that affect the heartbeat. Most of the time, the heart beats regularly, but occasionally the nerves develop a short circuit, triggering very rapid heartbeat known as PAT. Paroxysmal means sudden, atrial refers to the chamber of the heart where nerve impulses for the heartbeat begin, and tachycardia means fast heartbeat.

The attacks, though frightening, are usually not harmful. In rare cases, however, they can turn into a dangerous heart rhythm that can be fatal if not corrected.

Many over-the-counter medicines affect nerves and the activities they control, such as heartbeat and blood pressure.

Unless prescribed by your doctor, some medicines you should probably avoid are decongestants, cough, cold and sinus preparations, antihistamines, asthma and allergy medicines, diet pills, nose sprays and eye drops, especially products containing ephedrine, phenylephrine, phenylpropanolamine and pseudoephedrine. It's also possible that caffeine could make you more prone to tachycardia.

Q: Is there any treatment for canker sores? What about the new antiherpes drug, acyclovir?

A: Canker sore is the common term for a small, white, very painful sore inside the mouth. The medical term is aphthous ulcer. Canker sores are different from cold sores or fever blisters, which are caused by the herpes virus and usually occur on the lip.

A lot is still not known about what causes canker sores. Some scientists believe they're caused by a defective type of bacteria known as an L-form. Others blame them on stress or minor trauma to the delicate tissue lining the inside of the mouth. In many instances, they seem to break out for no apparent reason. Occasionally, multiple, recurrent canker sores are related to some other medical problems, such as Bechet's syndrome or Reiter's syndrome.

Although many remedies have been tried to treat canker sores, including zinc, vitamins, topical antibiotics and cortisone ointments, none seems to be universally effective. I don't think acyclovir would be helpful, either, because canker sores are not caused by herpes.

Perhaps the best treatment for occasional canker sores is a topical anesthetic such as viscous lidocaine, available by prescription, or an over-the-counter anesthetic for oral use, such as Kank-A or Anbesol. They don't cure the sore, but they ease the pain. The sore will heal by itself within a few days to a week. You should apply small amounts of these medicines to the sore with the enclosed applicator or cotton swab. Some people are sensitive to topical anesthetics, so you should stop using them if irritation develops.

Be sure to see your doctor for any sore that doesn't heal within a few weeks to make sure it isn't cancerous.