1. General Principles & Basic Science MCQs

A 4-year-old boy presents with progressive bowing of his legs. Laboratory evaluation demonstrates normal serum calcium, low serum phosphate, normal parathyroid hormone, and elevated alkaline phosphatase. Genetic testing reveals a mutation in the PHEX gene. Which of the following represents the primary pathophysiologic mechanism of this disease?

A 9-year-old boy presents with bilateral knee pain, a waddling gait, and difficulty running. Radiographs demonstrate delayed, irregular epiphyseal ossification and a classic "double-layered" patella on the lateral view. A mutation in which of the following genes is most commonly associated with this specific condition?

A 3-year-old girl with recurrent fractures and blue sclerae is started on an intravenous bisphosphonate protocol.

What is the primary cellular mechanism of action of this class of medication?

A 10-year-old girl presents for evaluation of delayed primary tooth loss and failure of permanent dental eruption. On physical exam, she is able to seamlessly appose her shoulders anteriorly in the midline.

The affected gene in this condition primarily orchestrates which of the following processes?

An 18-month-old child presents with severe clinical and radiographic signs of rickets, accompanied by total body alopecia. Laboratory evaluation reveals hypocalcemia, hypophosphatemia, and markedly elevated levels of 1,25-dihydroxyvitamin D. What is the most likely diagnosis?

A 65-year-old man presents with progressive anterior bowing of his right tibia and states his hats no longer fit. Biopsy of the affected bone reveals a mosaic pattern of lamellar bone with prominent cement lines. What is the primary initiating cellular event in the pathogenesis of this disease?

A newborn presents with severe anterior bowing of the lower extremities, hypoplastic scapulae, and respiratory distress due to tracheobronchomalacia. Genetic testing reveals a 46,XY karyotype, but the infant has unambiguous phenotypic female genitalia. A mutation in which gene is responsible for this condition?

A 14-month-old child fed exclusively on boiled cow's milk presents with extreme irritability, bleeding gums, and painful, swollen lower extremities. Radiographs show a dense zone of provisional calcification (white line of Frankel) and a radiolucent band directly beneath it (Trummerfeld zone). What specific biochemical process is impaired in this patient?

A 3-year-old boy presents with progressive bilateral bowing of his legs and a waddling gait. Laboratory studies reveal normal serum calcium, markedly decreased serum phosphate, normal PTH, and elevated alkaline phosphatase. Genetic testing confirms a mutation in the PHEX gene. Which of the following best describes the underlying pathophysiology?

A 3-year-old child with a history of recurrent fractures is initiated on cyclic intravenous pamidronate therapy.

What is the primary cellular mechanism of action of this medication?

A 32-year-old woman presents with knee pain. Radiographs show an eccentric, lytic epiphyseal-metaphyseal lesion of the distal femur extending to the subchondral bone. Biopsy shows multinucleated giant cells in a background of mononuclear stroma. Before surgical curettage, the multidisciplinary team recommends a targeted medical therapy. What is the mechanism of action of the most commonly used agent?

A 30-year-old woman presents with knee pain. Radiographs show an eccentric, purely lytic epiphyseal lesion in the distal femur extending precisely to the subchondral bone plate. A biopsy reveals multinucleated giant cells within a stroma of mononuclear cells. Which targeted therapy is most appropriate for locally advanced, unresectable cases of this tumor?

A 65-year-old man presents with generalized back pain, fatigue, and hypercalcemia. A skeletal survey demonstrates multiple "punched-out" lytic lesions in the skull and spine. Serum protein electrophoresis reveals a monoclonal IgG spike. What specific cell type is primarily responsible for the bone resorption seen in this disease process?

A 68-year-old man presents with an increasing hat size, unilateral hearing loss, and an anterolateral bowing deformity of his right femur. Radiographs show marked cortical thickening and coarse trabeculae. Which of the following laboratory profiles is most characteristic of this condition?

A 32-year-old woman presents with knee pain. Radiographs reveal an eccentric, lytic, epiphyseal lesion in the proximal tibia extending to the subchondral bone. Biopsy shows multinucleated giant cells in a background of mononuclear stromal cells. Which targeted therapy is most appropriate if the lesion is deemed unresectable?

A 70-year-old man presents with increasing hat size, hearing loss, and bowing of the tibiae. Laboratory tests show markedly elevated serum alkaline phosphatase but normal calcium and phosphorus levels. What is the primary cellular defect in the initial phase of this disease?

A 65-year-old woman presents with severe, progressive back pain. Radiographs show multiple "punched-out" lytic lesions in the skull and vertebrae. Serum protein electrophoresis shows an M-spike. Which pathway is primarily responsible for the prominent osteolytic lesions seen in this condition?

A 3-year-old boy presents with severe bowing of the legs and short stature. Labs show normal serum calcium, very low serum phosphorus, normal PTH, and elevated alkaline phosphatase. Genetic testing reveals a mutation in the PHEX gene. What is the core pathophysiology of this specific disease?

A 32-year-old woman presents with a lytic epiphyseal lesion of the proximal tibia. Biopsy confirms Giant Cell Tumor (GCT). For an unresectable lesion, which pharmacologic agent is most specifically targeted to the pathophysiology of this tumor?

A 12-year-old patient presents with multiple cafe-au-lait spots, axillary freckling, and Lisch nodules.

The genetic mutation responsible for this condition affects a protein that normally acts as a: