Questions: Lengthwise, long bone growth during infancy and youth is exclusively through - interstitial growth of the epiphyseal plates - the secretion of bone matrix into the medullary cavity - calcification of the matrix of the zone underlying articular cartilage - differentiation of osteoclasts into osteocytes

The answer is the first one: interstitial growth of the epiphyseal plates.

Explanation for each option:

1. Interstitial growth of the epiphyseal plates: This is the correct answer. Long bones grow in length during infancy and youth primarily through the interstitial growth of the epiphyseal plates, also known as growth plates. These plates are areas of cartilage located near the ends of long bones, and they allow for the lengthening of the bone as new cartilage is continuously formed and then ossified.

2. The secretion of bone matrix into the medullary cavity: This is incorrect. The medullary cavity is the central cavity of bone shafts where marrow is stored. Bone growth in length does not occur through the secretion of bone matrix into this cavity.

3. Calcification of the matrix of the zone underlying articular cartilage: This is incorrect. While calcification is a part of bone growth and development, the lengthwise growth of long bones is specifically associated with the activity at the epiphyseal plates, not the calcification of the matrix under articular cartilage.

4. Differentiation of osteoclasts into osteocytes: This is incorrect. Osteoclasts are cells that break down bone tissue, and osteocytes are mature bone cells. Osteoclasts do not differentiate into osteocytes; rather, osteoblasts become osteocytes. This process is not related to the lengthwise growth of bones.

In summary, long bone growth in length during infancy and youth is primarily due to the interstitial growth of the epiphyseal plates.