I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
Q- A 3-year-old boy is brought to the office because of a 2-day history of bulging of his left eye. He says his eye hurts. He has no history of major medical illness or recent trauma to the area, and he receives no medications. Vital signs are within normal limits. Physical examination shows exophthalmos of the left eye. MRI of the brain shows a 2-cm mass involving the ocular muscles of the left eye. A biopsy specimen of the mass shows malignant cells, some of which have striations. Which of the following is the most likely diagnosis?
(A) Neuroblastoma
(B) Pheochromocytoma
(C) Retinoblastoma
(D) Rhabdomyosarcoma
(E) Thyroid cancer
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Answer
The presentation of proptosis (exophthalmos), eye pain, and a mass involving the ocular muscles is highly suggestive of orbital rhabdomyosarcoma. The biopsy findings of malignant cells with striations strongly suggest rhabdomyosarcoma because the striations are indicative of skeletal muscle differentiation. Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children, and the orbit is a common site of involvement.
Neuroblastoma is common in young children but typically arises from the adrenal glands or paraspinal sympathetic ganglia. While it can metastasize to the orbit and cause proptosis, this is less likely without systemic symptoms or a primary tumor elsewhere.
Pheochromocytoma is rare in children. It arises from the adrenal medulla, causing symptoms like hypertension, which are not present here.
Retinoblastoma Affects the retina, not the ocular muscles and presents with leukocoria (white reflex in the pupil) or strabismus rather than proptosis and muscle involvement.
Thyroid cancer is not relevant in this case, as thyroid cancer does not typically involve orbital structures or present in young children.
The correct answer is D