While the external (lateral) epicondyle is the last apophyses to APPEAR on radiographs, this question is asking for which of the apopphyses is the last to FUSE. Therefore, the correct answer is the internal (medial) epicondyle, which is the last to FUSE at around 16 to 19 years. While the order of appearance is discussed more often, it is helpful to know the typical order and age of apophyseal closure when evaluating teenagers (often throwers) with atraumatic elbow pain.
An accepted mnemonic of the order of APPEARANCE of the individual ossification centers of the elbow is C-R-I-T-O-E: Capitellum, Radial head, Internal (medial) epicondyle, Trochlea, Olecranon, External (lateral) epicondyle.
Cheng et al performed a series to re-examine the sequence and pattern of elbow ossification based on a cross-sectional study of 3,154 elbow radiographs in children ranging in age from newborn to 17 years. The sequence of ossification in both boys and girls was found to be the same. The ages at which 50% of the girls were found to have positive radiologic ossification for each of these centers were ages 1, 5, 5, 9, 9, and 10 years, respectively. In boys, with the exception of the capitelum, an average delay of 2 years was found in each of the ossification centers, although the sequence remained similar.
Illustration A shows the different ossification centers. Illustration B shows the age of APPEARANCE and age of FUSION of the different ossification centers. It should be noted that appearance and fusion of the ossification centers do not correlate.
Beaty JH, Kasser JR. The elbow region: general concepts in the pediatric patient. In: Beaty JH, Kasser JR, eds. Rockwood and Wilkins' Fractures in Children. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:530-534.
Cheng JC, Wing-Man K, Shen WY, Yurianto H, Xia G, Lau JT, Cheung AY. A new look at the sequential development of elbow-ossification centers in children. J Pediatr Orthop. 1998 Mar-Apr;18(2):161-7.
PMID:9531396 (Link to Abstract)