The only comprehensible explanation is that the vocation that had burrowed in next to medicine had taken control, had insisted.
( ( ( The ( only ( comprehensible explanation ) ) ) ( is ( that ( ( ( the vocation ) ( that ( had ( burrowed ( in ( next ( to medicine ) ) ) ) ) ) ) ( had ( taken control ) ) ) ) ) ) ( , ( ( had insisted ) . ) ) )
(ROOT (S (NP (NP (DT The) (RB only) (JJ comprehensible) (NN explanation)) (SBAR (S (VP (VBZ is) (SBAR (IN that) (S (NP (NP (DT the) (NN vocation)) (SBAR (WHNP (WDT that)) (S (VP (VBD had) (VP (VBN burrowed) (PP (IN in) (NP (NP (JJ next)) (PP (TO to) (NP (NN medicine)))))))))) (VP (VBD had) (VP (VBN taken) (NP (NN control)))))))))) (, ,) (VP (VBD had) (VP (VBN insisted))) (. .)))
The only explanation is that nothing has taken control of medicine. 
( ( The ( only explanation ) ) ( ( is ( that ( nothing ( has ( taken ( control ( of medicine ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (JJ only) (NN explanation)) (VP (VBZ is) (SBAR (IN that) (S (NP (NN nothing)) (VP (VBZ has) (VP (VBN taken) (NP (NP (NN control)) (PP (IN of) (NP (NN medicine))))))))) (. .)))