added nbme12 tag, directly helps answer the Q:
A 2-year-old girl is brought to clinic because of a history of difficulty walking and intermittent crossing of the eyes. The abdomen is soft and nontender; a smooth, firm mass is palpated over the right upper quadrant. Neurologic examination shows rapid, chaotic, multidirectional saccades of the eyes. There are jerking motions of the extremities, which are exacerbated by intentional movement. There is truncal ataxia, and the patient is unable to sit or walk without assistance. Which of the following is the most likely underlying cause of this patient’s symptoms?
Answer: Neuroblastoma
Explanation: Opsoclonus myoclonus syndrome (OMS) typically presents in children between age 1
to 3 years with the acute onset of truncal ataxia, myoclonus, and opsoclonus. Opsoclonus is characterized by rapid, arrhythmic, multivectorial ocular saccades that occur without an intersaccadic interval. OMS often occurs as a paraneoplastic phenomenon of a neuroblastoma.