What is characterized by increased respiratory effort with a regular rhythm?

Hyperpnea is characterized by increased respiratory effort while maintaining a regular rhythm. This condition often occurs in response to increased metabolic demand, such as during exercise or in certain pathological states where the body requires more oxygen. The regular rhythm distinguishes hyperpnea from other respiratory patterns that may be irregular or gasping in nature.

Kussmaul's breathing, on the other hand, is a specific type of hyperventilation associated with metabolic acidosis, particularly diabetic ketoacidosis. It features deep, labored breaths but may not always present with a regular rhythm, making it distinct from hyperpnea.

Conditions like pneumothorax and atelectasis involve alterations in lung mechanics but do not primarily manifest as increased respiratory effort with a regular rhythm. Pneumothorax can lead to uneven or diminished breath sounds and may cause difficulty in breathing due to a collapsed lung, while atelectasis refers to the collapse of lung tissue that can lead to reduced airflow, again not characterized by increased effort in a regular rhythm.

Thus, increased respiratory effort with a regular rhythm is best described by hyperpnea, making it the most appropriate choice.