Which statement correctly describes the origin, insertion, and action of the vastus intermedius?

• A. Origin: anterior/lateral surface of the femur. Insertion: patella; tibial tuberosity via patellar ligament. Action: knee extension.
• B. Origin: medial femoral condyle. Insertion: patella. Action: knee extension.
• C. Origin: posterior femur. Insertion: calcaneus. Action: plantarflexion.
• D. Origin: anterior/lateral surface of the femur. Insertion: patella; tibial tuberosity via patellar ligament. Action: knee extension.

Answer: (A)

Vastus intermedius is one of the quadriceps muscles lying on the front of the thigh, deep to rectus femoris. It originates along the anterior and lateral surfaces of the femoral shaft, and its fibers converge into the quadriceps tendon to insert on the patella, with the patellar ligament continuing to the tibial tuberosity. Its primary action is knee extension, working with the other quadriceps heads to straighten the leg.

The description that places the origin at the medial femoral condyle would correspond to a different muscle (vastus medialis). Descriptions that insert on the calcaneus and imply plantarflexion refer to calf muscles (like gastrocnemius/soleus) rather than the quadriceps.