Which of the following is a sign of inadequate breathing in an infant?

Expiratory grunting is a significant sign of inadequate breathing in an infant. It typically indicates that the infant is experiencing respiratory distress and is attempting to exhale forcefully against a closed airway. This sound occurs as the infant tries to maintain end-expiratory lung volume in an effort to improve oxygenation and ventilate effectively despite impaired lung function. Grunting is a compensatory mechanism that indicates the body is struggling to meet its oxygen demands and is often associated with conditions such as pneumonia, atelectasis, or other forms of respiratory distress.

In contrast, the other signs listed do not directly indicate inadequate breathing. Sunken fontanelles can suggest dehydration or increased intracranial pressure rather than directly reflect respiratory distress. Abdominal breathing is common in infants and may be a normal finding, especially in infants who are calm and not in distress. A heart rate of 130 beats per minute falls within the normal range for infants, where normal rates can vary between 100 to 160 beats per minute when at rest. Therefore, while these signs may indicate other health concerns, expiratory grunting specifically points to inadequate breathing and respiratory compromise.