Cystic fibrosis (CF) is suspected in a toddler. Which test is essential in establishing this diagnosis?

This test measures the amount of chloride in the sweat, which is abnormally high in people with cystic fibrosis (CF). CF is an inherited disorder that affects the cells that produce mucus, sweat, and digestive juices.

Choice A is wrong because bronchoscopy is a procedure that allows the doctor to examine the airways and lungs, but it is not essential for diagnosing CF.

Choice B is wrong because serum calcium is a blood test that measures the level of calcium in the blood, which is not related to CF.

Choice C is wrong because urine creatinine is a test that measures the amount of creatinine in the urine, which reflects the kidney function, but it is not relevant to CF.

Normal ranges for sweat chloride test are:

• Less than 40 millimoles per liter (mmol/L) for children and adults
• Less than 30 mmol/L for infants younger than 6 months

A sweat chloride level of more than 60 mmol/L is considered positive for CF.

This is because toddlers continue to have the short, straight internal ear canal of infants.

The lymphoid tissue of the tonsils and adenoids continues to be relatively large. These two anatomic conditions combine to predispose the toddler to frequent infections.

Choice A is wrong because respirations are abdominal. This does not affect the susceptibility to infection.

Choice B is wrong because pulse and respiratory rates are slower than those in infancy. This also does not affect the susceptibility to infection.

Choice C is wrong because defense mechanisms are less efficient than those during

infancy. This is not true, as the defense mechanisms are more efficient compared with those of infancy.