Final answer:
The muscle responsible for MTP flexion is the plantar group of intrinsic muscles in the foot, including the flexor digitorum brevis.
Step-by-step explanation:
The muscle responsible for MTP flexion is the plantar group of intrinsic muscles in the foot. This group includes several muscles that work together to flex the toes at the metatarsophalangeal (MTP) joints. The muscles in this group abduct and flex the middle toes and extend the middle toes at the interphalangeal (IP) joints. These movements are important for activities like walking and running.
For example, the flexor digitorum brevis is one of the muscles in the plantar group. It originates from the calcaneal tuberosity (which is the answer choice 1), and it inserts into the bases of the 2nd-5th phalanges (which is the answer choice 2). When the flexor digitorum brevis contracts, it flexes the MTP joints of the middle toes.
The other answer choices are not correct for the muscle responsible for MTP flexion. The medial plantar nerve (answer choice 3) is a nerve that provides innervation to the plantar muscles, including the ones responsible for MTP flexion. However, it is not the muscle itself. The correct answer is the plantar group of intrinsic muscles.