Unit 4B Review

Anatomy

200

_{A 22 year old male presents with pain and swelling in the upper gluteal cleft for 4 days. He reports tenderness with sitting. On exam, there is a 3-cm erythematous, fluctuant mass with a small midline pit and drainage superior to the gluteal cleft. His temp is 39.9°C (100.2°F). Which is the most appropriate initial management?}

_{a. oral antibiotics alone}

_{b. incision and drainage of lesion}

_{c. wide surgical excision of the sinus tract}

_{d. topical corticosteroids}

_{e. radiation}

_{B. Incision and drainage} 

_{-- Pilonidal disease is a cyst, abscess, or sinus due to a reaction to a disrupted hair follicle pulled into subcutaneous tissue.}

_{Other notes:} 

_{- Antibiotics alone are not enough}

_{- Surgical excision is performed later or with recurrent disease}

_{- M>F, young age, obese, prolonged sitting, and coarse body hair increases risk}

201

_{An 18 year old cyclist complains of weakness in the right hand after 25 days of cross-country cycling. She reports numbness and tingling on the medial side of the hand. Physical exam shows 4th/5th digits hyperextended at MCP and flexed at PIP joints.} 

_{What nerve is damaged?} 

_{What are the main hand muscles affected?} 

_{Ulnar nerve - "claw hand"} 

_{-- This nerve falls into the Guyon's canal between the pisiform & hook of hamate}

_{Paralysis is caused to the intrinsic hand muscles}

202

_{A 25 year old man presents after a posterior thigh laceration from a glass injury. On exam, he is unable to flex his knee or extend his hip. Sensation of the posterior hip is intact, but sensation over most of the leg and foot is decreased. The injured nerve most likely arises from which of the following spinal nerve roots?}

_{a. L2-L4} 

_{b. L3-L5}

_{c. L4-S1}

_{d. L4-S3}

_{e. S2-S4}

_{Extra: what muscles are affected? what are the other incorrect nerves referenced?}

_{D. L4-S3 = Likely injured tibial division of the sciatic nerve}

_{--- muscles affected = biceps femoris (long head), semitendinosis, semimembranosis = actions of knee flexion and hip extension}

_{--- Others =} 

_{A. L2-L4 ~ femoral n. (anterior thigh)} 

_{B. L3-L5 ~ obturator n. (medial thigh adductors)} 

_{C. L4-S1 ~ sup. gluteal n. (gluteus med/min = trendelenburg)}

_{E. S2-S4 ~ pudendal n. (perineal sensation and sphincter control)}

203

_{A 21-year-old soccer player presents after twisting his knee while pivoting during a game. He reports hearing a “pop” followed by rapid knee swelling. Physical exam shows increased anterior translation of the tibia relative to the femur during the Lachman test. MRI confirms a tear of the Anterior cruciate ligament.}

_{Which of the following movements is primarily prevented by this ligament?}

_{A. Posterior translation of the tibia relative to the femur}

_{B. Anterior translation of the tibia relative to the femur}

_{C. Medial displacement of the patella}

_{D. Hyperextension of the knee through posterior capsule tension}

_{E. Lateral displacement of the tibia during knee flexion}

_{B. Anterior translation of the tibia relative to the femur}

_{--- ACL restrains forward movement of the tibia, classic non-contact pivot on planted foot}

_{--- Others:} 

_{a. posterior translation = PCL} 

_{c. medial displacement = medial patellofemoral ligament}

_{d. hyperextension = posterior joint capsule and surrounding ligaments}

_{e. lateral displacement = LCL}

Muscles - Development, Repair, & Build

200

_{During embryonic development, muscle progenitor cells migrate from the dermomyotome into the developing limb buds. This migration depends on signaling molecules that activate the receptor tyrosine kinase c-Met on muscle precursor cells.}

_{A researcher creates a mouse model in which the ligand for the c-Met receptor is knocked out. Which of the following developmental abnormalities would most likely result?}

_{A. Failure of limb muscle formation due to impaired migration of myogenic precursor cells}

_{B. Failure of vertebral body formation due to abnormal sclerotome differentiation}

_{C. Absence of dermis formation due to defective dermatome development}

_{D. Absence of neuromuscular junctions due to failure of acetylcholine receptor clustering}

_{E. Failure of neural crest cell migration leading to craniofacial defect}

_{A. Failure of limb muscle formation due to impaired migration of myogenic precursor cells}

_{--- cMet is the ligand for HGF critical for muscle precursors ability to migrate into the limb for muscle formation} 

_{--- Others:}

_{b. vertebral body forms from the sclerotome influenced by Shh}

_{c. absence of dermis formation from the dermatome influenced by Wnt proteins}

_{d. absence of NMJ due to failure of AchR clustering requiring MuSK signaling}

_{e.  Failure of neural crest cell migration leading to craniofacial defects -- not affected by cMet}

201

_{A skeletal muscle biopsy from a patient with exercise‑induced muscle weakness is examined by transmission electron microscopy (TEM). The image shows marked widening of the I‑bands and Z‑line displacement, with no change in A‑band width. Myosin‑II thick filaments appear intact, but thin filament anchoring seems disrupted.}

_{Which molecular abnormality BEST explains these findings?}

_{A. A defect in M‑line protein assembly
B. A mutation impairing tropomyosin’s ability to block myosin binding sites
C. Loss of Ca²⁺ release from the sarcoplasmic reticulum due to faulty triad function
D. A defect in α‑actinin localization at the Z‑line
E. Myosin‑II heavy‑chain mutation affecting the A‑band}

_{D. A defect in α‑actinin localization at the Z‑line}

_{--- Z-line anchors + ends of thin filaments via a-actinin -- loss of this = thin filament instability & widening of I-band & Z-line misalignment} 

_{--- Others:} 

_{a. defect in M-line assembly = anchors thick filaments at the center of the H-zone, defect here would affect A/H bands (not seen)} 

_{b. impaired tropomyosin = blocks actin binding / would cause abnormal contraction not widening of bands}

_{c. loss of Ca2+ release from SR = would cause functional weakness not alternation of sarcomere composition}

_{e. myosin-II heavy chain mutation affecting A-band = A-band is stated to be unchanged}

202

_{A researcher is studying skeletal muscle fibers using electron microscopy. In a longitudinal section, she notes long cylindrical cells containing numerous parallel myofibrils, each composed of repeating sarcomeres. She identifies a structure formed by a T‑tubule flanked on each side by terminal cisternae of the sarcoplasmic reticulum.}

_{Which of the following BEST describes the function of this structure?}

_{A. Stabilizes the thin filaments by binding to their (+) ends
B. Blocks myosin binding sites on actin at rest
C. Anchors thick filaments at the center of the sarcomere
D. Contains the Ca²⁺‑binding complex that shifts tropomyosin
E. Couples membrane depolarization with intracellular Ca²⁺ release}

_{E. Couples membrane depolarization with intracellular Ca2+ release}

_{--- Made of a T-tubule + 2 terminal cisternae of SR which couples nerve stimulation with Ca2+ release for muscle contraction}

_{--- Others:} 

_{a. a-actinin binding thin filament = z-line not triad}

_{b. blocking myosin binding = tropomyosin}

_{c. anchors thick filaments = m-line not triad}

_{d. Ca2+ binding complex = troponin}

203

_{A 22-year-old soccer player sustains a grade II strain of the hamstring during sprinting. Within 24 hours, he presents with localized pain, swelling, and weakness. Which of the following cellular events is MOST critical for successful skeletal muscle regeneration in the days following the injury?}

_{A. Activation of chondrocytes to form fibrocartilage at the site of injury
B. Differentiation of fibroblasts into myofibroblasts to produce dense scar tissue
C. Activation and proliferation of satellite cells to form new myotubes
D. Inhibition of inflammation to prevent neutrophil-mediated tissue damage
E. Fusion of osteoprogenitor cells to form new multinucleated fibers}

_{C. Activation and proliferation of satellite cells to form new myotubes
--- satellite cells are resident skeletal muscle stem cells that activate after injury to fuse and form new myotubes for repair}

_{--- Others:} 

_{a. chondrocytes = not part of skeletal muscle healing (tendon/ligament injury instead)}

_{b. fibroblasts = scar tissue NOT restoring contractile property}

_{d. inhibit inflammation = NECESSARY to clear debris and signal satellite cell activation}

_{e. osteoprogenitor cell fusion = form bone not muscle (this would be abnormal repair)}

Muscle/CT infections

200

_{A 54‑year‑old man develops sudden, severe pain at the site of a leg laceration he sustained the day before. The area is pale, tense, and exquisitely tender. Within hours, the skin becomes bronze then purplish‑red. Imaging reveals gas in the soft tissues. Gram stain of wound exudate shows large gram‑variable rods. Which toxin is primarily responsible for shock and systemic vascular collapse in this condition?}

_{A. Tetrodotoxin
B. Alpha‑toxin
C. Theta‑toxin
D. Shiga toxin}

_{Hint: pick out the bacteria information, what toxins does this make? what are the toxins functions?} 

_{C. Theta‑toxin}

_{--- Clostridial gas gangrene produces several toxins; theta contributes heavily to systemic shock while alpha toxin causes myonecrosis and impairs contractility}

_{--- Others:} 

_{a. tetrodotoxin = in marine bacteria (like some Vibrio spp. [not vulnificus], pufferfish) causing flaccid paralysis}

_{b. alpha = myonecrosis}

_{d. shiga toxin = from shigella or E. coli causing cell death}

201

_{A 41‑year‑old man with alcoholic liver disease develops rapidly worsening leg pain after sustaining a minor abrasion while wading in coastal water. Examination shows violaceous bullae and necrotizing soft‑tissue infection. His condition deteriorates rapidly toward septic shock. Which organism is the most likely cause?}

_{A. Aeromonas hydrophila
B. Vibrio vulnificus
C. Streptococcus pyogenes
D. Clostridium septicum}

_{B. Vibrio vulnificus}

_{--- associated with necrotizing fasciitis and blistering skin, especially with iiver disease patients who are exposures to water sources}

_{--- Others:} 

_{a. A. hydrophila = associated with fresh water}

_{c. Strep pyogenes = GAS, not seawater exposure}

_{d. C. septicum = causes spontaneous gas gangrene often associated with GI malignancy}

202

_{A 9‑year‑old child presents with diffuse thigh pain, weakness, and tenderness on palpation. MRI reveals muscle inflammation without abscess formation. Blood cultures grow gram‑positive cocci in clusters. The organism most commonly causes myositis via hematogenous spread. Which organism is the most likely pathogen?}

_{A. Streptococcus pyogenes
B. Staphylococcus aureus
C. Clostridium perfringens
D. Mycobacterium tuberculosis}

_{B. S. aureus}

_{--- most commonly spreads hematogenously to muscle to cause myositis; most frequent isolated pathogen in peds cases without trauma}

_{--- Others:} 

_{a. S. pyogenes = GAS, more implicated in necrotizing fasciitis}

_{c. C. perfringens = leading cause of traumatic gas gangrene after deep penetrating injury}

_{d. M. tuberculosis = not common cause of acute infective myositis in children as presented}

203

_{A 67‑year‑old woman with colon adenocarcinoma presents with abrupt, excruciating thigh pain and rapidly progressive soft‑tissue destruction. She is confused and hypotensive. Blood cultures are positive before local symptoms appear. Which organism is the most likely cause?}

_{A. Clostridium septicum
B. Clostridium perfringens
C. Staphylococcus aureus
D. Escherichia coli}

_{A. C. septicum}

_{--- spontaneous (non-traumatic) gas gangrene is classic with this organism}

_{--- Others:} 

_{b. C. perfringens = traumatic gas gangrene not spontaneous presentation linked to GI cancer}

_{c. S. aureus = does not produce rapid-gas forming myonecrosis}

_{d. E. coli = can cause bacteremia but gas gangrene / GI malignancy is classic with septicum}

Bone Tumors

200

_{A 14‑year‑old boy presents with nighttime knee pain relieved by NSAIDs. X‑ray shows a 1 cm intracortical radiolucent nidussurrounded by reactive sclerosis in the proximal tibia.
What is the most likely diagnosis?}

_{A. Osteoblastoma
B. Enchondroma
C. Osteoid osteoma
D. Ewing sarcoma}

_{C. Osteoid osteoma}

_{--- < 2 cm, pain relieved by NSAIDs, < 25 years old, intracortical} 

_{--- Others:} 

_{a. osteoblastoma = larger, more common in spine, pain NOT relieved by NSAIDs}

_{b. enchondroma = chondroid calcification, in hands and feet often, not cortical} 

_{d. ewing sarcoma = diaphyseal, onion-skin periosteal reaction, systemic symptoms}

201

_{A 16‑year‑old boy has progressive knee pain. X‑ray shows a metaphyseal lesion with sunburst periosteal reaction and cortical destruction. Biopsy shows pleomorphic malignant cells producing osteoid.
Which gene is most commonly mutated in this tumor?}

_{A. RB
B. APC
C. NF1
D. BRCA1}

_{A. RB}

_{--- RB mutations are present in 70% osteosarcomas which arise in metaphyses of long bones and produce malignant osteoid}

_{--- Others:} 

_{b. APC = colorectal cancer}

_{c. NF1 = neural tumors} 

_{d. BRCA1 = breast / ovarian cancers}

202

_{A 32‑year‑old woman presents with knee pain and swelling. Imaging shows a well‑defined, lytic, eccentric lesion in the epiphysis of the distal femur with a soap‑bubble appearance. Biopsy shows numerous osteoclast‑like giant cells evenly distributed in a mononuclear stromal background.
What is the most likely diagnosis?}

_{A. Aneurysmal bone cyst
B. Giant cell tumor of bone
C. Chondromyxoid fibroma
D. Osteochondroma}

_{B. Giant cell tumor of bone}

_{--- in skeletally mature adults, common in knee, soap bubble radiology, RANKL-expressing stromal cells with giant cells}

_{--- Others:} 

_{a. aneurysmal bone cyst = usually metaphyseal and cystic with fluid}

_{c. chrondomyxoid fibroma = chondromyxoid matrix not giant cells}

_{d. osteochondroma = metaphyseal exostosis with cartilage cap not epiphyseal or lytic}

203

A 13‑year‑old boy presents with leg pain, fever, and leukocytosis. X‑ray shows a diaphyseal destructive lesion with onion‑skin periosteal reaction. Biopsy shows small round blue cells.
Which chromosomal translocation is associated with this tumor?

A. t(9;22)
B. t(15;17)
C. t(8;14)
D. t(11;22)

_{D. t(11;22)}

_{--- Ewing sarcoma is defined by this translation which occurs in diaphyses of long bones with onion-skin periosteal reaction}

_{--- Others:} 

_{a. t(9;22) = Philadelphia chromosome (CML)}

_{b. t(15;17) = acute promyelocytic leukemia}

_{c. t(8;14) = Burkitt lymphoma}

Cause & Effect of Muscle Movement - Contraction & Gait

200

_{A 54‑year‑old man presents with progressive difficulty walking over the past year. His wife reports that he “walks like he’s being pulled forward” and has trouble initiating movement unless he takes several short preparatory steps. On exam, his gait is narrow-based, with short, shuffling steps and decreased arm swing bilaterally. When asked to turn, he rotates his body “en bloc.” Strength and sensation in the lower limbs are normal.}

_{Which of the following best explains the underlying mechanism of this patient’s gait abnormality?}

_{A. Loss of sensory feedback from muscle spindles producing bilateral high‑stepping gait
B. Weakness of hip abductors causing pelvic drop during the swing phase
C. Basal ganglia dysfunction impairing initiation and scaling of movement
D. Lesion of the common fibular nerve causing inability to dorsiflex during swing
E. Frontal lobe motor planning deficit producing magnetic gait}

_{Additionally explain wrong answers.}

_{C. Basal ganglia dysfunction impairing initiation and scaling of movement}

_{--- Parkonsonian gait = slow, shuffling, narrow-base, difficulty starting, "en-bloc" turning}

_{--- stems from BASAL GANGLIA dysfunction leading to the impaired initiation and reduced step size} 

_{--- Others:} 

_{a. loss of sensory feedback = bilateral high stepping gait (tabetic)} 

_{b. hip abductor weakness = trendelenburg}

_{d. common fibular nerve lesion = foot drop = high stepping} 

_{e. frontal motor deficit = magnetic or apraxic gait}

201

_{A 24 year old hiker pauses during an ascent in the Canadian Rockies. While standing on a firm surface, he becomes lightheaded and loses balance. Moments earlier, he was standing with his knees locked in extension while admiring the view. Which explains the physiological basis for the loss of balance?} 

_{a. reduced muscular activity at the ankle decreasing venous return, predisposing to syncope}

_{b. loss of tonic activation in hip extensors reducing posterior pelvic stability}

_{c. increasing energy expenditure at the knee causing rapid fatigue and orthostatic instability}

_{d. lack of visual input leading to excessive body sway and compensatory collapse}

_{e. failure of passive ligamentous support at the ankle increasing mediolateral sway}

_{--- additional: why are the wrong choices wrong?}

_{A. reduced muscular activity at the ankle decreasing venous return, predisposing to syncope}

_{--- muscle energy in a normal stance is only expended at the ankle BUT reduced muscular activity leads to reduced venous return}

_{--- Others:} 

_{B. hip is most stable in static stance}

_{C. energy not expended at the knee in normal stance}

_{D. visual cues were not deprived}

_{E. ankle IS least stable joint but it isn't ligament failure as cause of sway}

202

_{A 32‑year‑old woman undergoes laparoscopic surgery. During the procedure, insufflation of the abdomen stretches portions of her gastrointestinal tract. Soon after, certain segments of her bowel show increased contractile activity, despite no direct increase in neural firing to those segments. Laboratory analysis of a biopsy later reveals elevated myosin light chain phosphorylation, normal myosin light chain phosphatase (MLCP), and only a modest rise in intracellular Ca²⁺.}

_{Which mechanism most likely accounts for the observed increase in contractile force?}

_{A. Activation of stretch‑operated ion channels producing spontaneous depolarization}

_{B. Recruitment of multiple motor units producing stronger contractions}

_{C. Troponin‑C–mediated enhancement of actin–myosin interactions}

_{D. Increased sensitivity of myosin light chain kinase (MLCK) to Ca²⁺‑calmodulin}

_{E. Ca²⁺‑induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum as the major Ca²⁺ source}

_{Additionally, explain the wrong choices.}

_{D. Increased sensitivity of myosin light chain kinase (MLCK) to Ca²⁺‑calmodulin}

_{--- smooth muscle contraction strength can be increased by increased by Ca increase or MLCK sensitivity causing more phosphorylation} 

_{--- Others:} 

_{a. activate ion channels = would increase Ca2+ entry and cause LARGE calcium increase}

_{b. recruitment of multiple motor units = skeletal muscle mechanism}

_{c. troponin C = skeletal muscle mechanism}

_{e. Ca2+ induced Ca2+ release from SR = smooth muscle relies mainly on extracellular Ca2+} 

203

_{A 34-year-old man presents to clinic after a traumatic injury to the posterior compartment of his leg. On physical exam, he has difficulty pushing off the ground during the terminal stance phase of gait. Muscle biopsy shows intact actin and myosin filaments but impaired release of calcium from the sarcoplasmic reticulum, leading to decreased activation of the contractile apparatus.}

_{Which of the following proteins is most directly responsible for initiating skeletal muscle contraction after calcium is released?}

_{A. Tropomyosin
B. Myosin ATPase
C. Titin
D. Troponin I
E. Troponin C}

_{E. Troponin C}

_{--- binds Ca2+ released from the SR during excitation-contraction coupling leading to open binding sites for actin and cross-bridge cycling}

_{--- Others:} 

_{a. tropomyosin = blocks actin binding sites but does not bind calcium}

_{b. myosin ATPase = hydrolyzes ATP for the power stroke (on myosin head)} 

_{c. titin = structural protein anchoring myosin to z-line, contributes to passive tension}

_{d. troponin I = inhibitory subunit preventing actin-myosin interaction}