1. d. Joint stiffness can be congenital or acquired. Arthrogryposis refers to joint contractures observed at birth. Immobility interferes with limb development and causes joint fixation and formation of pterygia. Acquired inflammatory conditions such rheumatoid arthritis or osteoarthritis lead to joint pain and stiffness. On the other hand, patients with Marfan syndrome or homocystinuria have increased mobility of the joints.

2. c.These are osteoblasts with characteristic plasmacytoid cytoplasm. The nuclei, however, do not look like plasma cells. Osteoclasts can also be seen in bone aspirates but have multiple nuclei. Langerhans and Gaucher cells have characteristic morphology not observed in this smear.

3. b. Short stature with kyphosis and delay in ossification is a feature of congenital hypothyroidism. Pseudohypoparathyroidism is caused by defects in receptors and leads to obesity, short stature, and shortening of metacarpals, metatarsals, and phalanges. Hypoparathyroidism occurs in DiGeorge syndrome and also can be associated with short stature. Congenital adrenal hyperplasia, growth hormone deficiency, hypopituitarism resulting from adenomas can all lead to retardation of growth in children.

4. a. Rickets or osteomalacia results from conditions leading to impaired availability of vitamin D, decreased alkaline phosphatase, or increased loss of phosphates in urine. On the other hand, scurvy is characterized by inability to form extracellular collagen, which leads to osteopenia, demineralization, subperiosteal hemorrhage, and microfractures.

5. e. McCune-Albright syndrome compromises polyostotic fibrous dysplasia, skin pigmentation, and precocious puberty. Somatic mutations in the G signal transduction protein have been found in persons with this syndrome. The syndrome also causes variety of bony and endocrine abnormalities including hyperthyroidism.

6. d. Tall stature can result from growth hormonesecreting tumors, thyrotoxicosis, homocystinuria, and Klinefelter and Marfan syndromes. Patients with Turner syndrome are typically short females with broad chest, wide spacing of the nipples, and congenital lymphedema of the neck, and have 45XO chromosomal pattern.

7. c. Combination of short extremities, narrow thorax, and frontal bossing with cloverleaf-like skull configuration is consistent with a skeletal dysplasia. Patients with skeletal dysplasia usually die of respiratory failure due to lung hypoplasia.

8. e. CD99 positivity is traditionally associated with Ewing sarcoma/PNET. However, some lymphomas, small cell osteosarcoma, and monophasic synovial osteosarcoma can all stain with CD99.

9. b. Elastofibroma is a benign, poorly circumscribed reactive tumor condition occurring exclusively in the scapular region of elderly people and is characterized by large numbers of prominent elastic fibers.

10. c. Osteosarcoma is the most likely diagnosis in this age group and can have cartilaginous differentiation. Thin osteoid can be seen in between the tumor cells in the left upper quadrant of the first image. Chondrosarcoma and malignant fibrous histiocytoma are rare in this age group.

11. d. Grading of sarcomas emphasizes tumor differentiation, number of mitotic figures, and tumor necrosis. For example, tumors with >5 mitosis/10 HPF and >15% necrosis are considered high grade or grade III. Certain tumors are by definition high grade, such as alveolar rhabdomyosarcoma.

12. d. Nodular fasciitis grows rapidly after trauma and usually affects children and young adults between 20 to 40 years of age. The tumor can have spontaneous regression and does not usually recur after excision. It can involve the skull bones but is considered a reactive nonneoplastic process.

13. c.The tumor is composed of lipoblasts in a myxoid background with delicate vasculature and is therefore a myxoid liposarcoma. Translocation t(12;16) is found in >90% of myxoid liposarcoma and leads to fusions of CHOP-FUS genes. In rare cases t(12;22)(q13;q12) has been described with fusion of CHOP-EWS ge

14. a. CD31, CD34, factor VIII, and Ulex europaeus lectin are endothelial markers that can be used to stain hemangiomas and other tumors. GLUT-1 is another endothelial marker that is specific for infantile hemangiomas. D2-40 stains lymphatic vessels and is not a general endothelial marker.

15. d. Malignant fibrous histiocytoma of the pleomorphic type is the most common postradiation sarcoma in adults and occurs after an interval of 10 to 12 years. It can also arise in chronic ulcers and scars. Other conditions can also develop in sites of prior radiation, including reactive spindle cell proliferation, fractures, osteosarcoma, and fibrosarcoma.

16. b. Aneurysmal bone cyst is usually seen in 1 to 20 years of age and occurs mainly in vertebrae and flat bones. Fibroblasts and giant cells in septa that separate cystic spaces are characteristic of aneurysmal bone cyst. On the other hand, solitary or unicameral bone cyst is lined by a slender fibrous membrane that appears different from that of aneurysmal bone cyst.