Foundational Science and Mechanisms of Disease

A 68-year-old man comes to the clinic due to a 1-year history of worsening intention tremor and unsteadiness while walking. He also reports recent difficulties with short-term memory and executive function, such as managing his finances. His medical history is notable for hypertension. His family history is significant; his daughter is healthy, but his 8-year-old maternal grandson has been diagnosed with intellectual disability, a long face, prominent ears, and macroorchidism. On physical examination, the patient exhibits a bilateral intention tremor, broad-based ataxic gait, and symmetrical parkinsonian features including mild rigidity. Magnetic resonance imaging (MRI) of the brain reveals generalized cerebral atrophy and symmetrically increased T2-weighted signal in the middle cerebellar peduncles. Which of the following best describes the underlying molecular mechanism responsible for this patient's current neurologic symptoms?

A 28-year-old man with recently diagnosed Crohn disease comes to the emergency department due to a 3-day history of high fever, chills, and severe fatigue. He also reports painful sores in his mouth. Four weeks ago, his gastroenterologist started him on azathioprine and a tapering dose of prednisone to manage a severe disease flare. His temperature is $39.2^\circ\text{C}$ ($102.6^\circ\text{F}$), blood pressure is $110/70\text{ mm Hg}$, and pulse is $115/\text{min}$. Physical examination shows pale conjunctivae and multiple aphthous ulcers on the buccal mucosa. Laboratory studies show:

Which of the following genetic mechanisms best explains this patient's current presentation?

Match each classic genetic principle to its defining clinical or pathophysiological description.

A 3-week-old male infant is evaluated in the pediatric genetics clinic for profound central hypotonia, a poor suck reflex requiring nasogastric tube feeding, and cryptorchidism. The clinical team strongly suspects Prader-Willi syndrome. Arrange the following diagnostic modalities in the correct sequential order recommended to first confirm the clinical diagnosis and subsequently determine the exact underlying molecular etiology, proceeding from the initial confirmatory test to the evaluation for the rarest genetic mechanism.

A 45-year-old woman undergoes a hemicolectomy for a newly diagnosed right-sided colon adenocarcinoma. Her family history is significant for a mother who had endometrial cancer at age 49. The physician suspects Lynch syndrome (hereditary nonpolyposis colorectal cancer) and initiates a stepwise molecular and genetic diagnostic workup. Assuming each diagnostic test yields findings that require proceeding to the next sequential step, arrange the following management actions in the correct chronological order.

A 54-year-old man with a 10-year history of uncontrolled gastroesophageal reflux disease undergoes an upper gastrointestinal endoscopy. A biopsy of the distal esophagus is obtained. Histologic examination of the tissue reveals the replacement of the normal stratified squamous epithelium with nonciliated, mucin-producing columnar epithelium containing goblet cells. No cytologic atypia is observed. Which of the following cellular processes best describes this histologic finding?

A 55-year-old man experiences a myocardial infarction, resulting in abrupt cessation of blood flow to a portion of the left ventricle. This ischemic event triggers a predictable sequence of subcellular changes leading to reversible cellular injury. Arrange the following pathophysiological events of ischemic cellular injury in the correct chronological order, from earliest to latest.

A 32-year-old man comes to the clinic to review the results of his recent laboratory tests. He originally presented for a well-patient examination, during which firm, nodular masses were noted on his bilateral Achilles tendons. A fasting lipid panel at that time showed a low-density lipoprotein (LDL) cholesterol level of 315 mg/dL. Due to a strong family history of early-onset coronary artery disease, he was referred for targeted genetic testing. The results reveal a heterozygous gain-of-function mutation in the PCSK9 gene.

During the visit, the patient mentions that his maternal cousin, who is 35 years old, has the identical clinical phenotype consisting of tendon xanthomas and severely elevated LDL cholesterol levels. The cousin also recently underwent targeted genetic testing, but his results showed a heterozygous loss-of-function mutation in the LDLR gene.

Which of the following genetic principles best explains the presence of an identical clinical phenotype in these two relatives despite having mutations in entirely different genes?

An outpatient evaluation is conducted for a $24$-year-old male who reports a two-month history of episodic, severe headaches accompanied by palpitations, profound diaphoresis, and anxiety. During the visit, his blood pressure is measured at $185/110$ mm Hg with a heart rate of $115$/min. Physical examination is notable for an unusually tall and slender body habitus with arachnodactyly. Examination of the oral cavity reveals multiple firm, painless, yellow-white nodules distributed across the lips and the anterior tip of the tongue. Palpation of the anterior neck identifies a firm, $2$-cm nodule in the right thyroid lobe. Diagnostic laboratory studies show markedly elevated plasma free metanephrines. A review of his family history indicates that his mother passed away from a rapidly progressive neck tumor at age $35$, and his $12$-year-old brother recently underwent an elective prophylactic total thyroidectomy. Based on these clinical findings, the genetic defect responsible for this patient's condition causes which of the following molecular alterations?

A 62-year-old woman is managed in the intensive care unit for gram-negative bacteremia and distributive shock. Despite appropriate antibiotic therapy and aggressive fluid resuscitation, she experiences prolonged episodes of severe hypotension over a 36-hour period. Her serum creatinine level increases rapidly from a baseline of 0.8 mg/dL to 3.2 mg/dL. Urinalysis reveals the presence of numerous muddy brown granular casts. The patient's renal dysfunction is primarily driven by ischemic damage to the proximal renal tubules. Which of the following intracellular events marks the definitive transition from reversible adaptation to irreversible cell injury in this patient's affected renal tubules?

A 14-month-old girl is evaluated in the pediatric genetics clinic for poor weight gain, recurrent respiratory infections, and greasy, foul-smelling stools. Physical examination reveals a weight below the 3rd percentile, a prominent forehead, a triangular face, and noticeable left-sided limb hypoplasia. Sweat chloride testing is elevated. Molecular genetic testing reveals the patient is homozygous for the $\Delta F508$ mutation in the CFTR gene. Carrier screening of both parents, along with microsatellite analysis confirming paternity, shows the mother is a heterozygous carrier of the $\Delta F508$ mutation, whereas the father has two normal CFTR alleles. What specific underlying molecular event is most likely responsible for this child's condition?

Match each clinical presentation to the underlying cellular adaptation mechanism it best represents.

A 52-year-old male presents to the transplant nephrology clinic for evaluation of a low-grade fever and a significant decrease in his daily urine output. He received a living-donor kidney transplant one month ago. His vital signs are stable, but physical examination reveals mild tenderness over the allograft site in the right lower quadrant. Recent laboratory tests show an acute elevation in his serum creatinine from a baseline of 1.3 mg/dL to 2.9 mg/dL. Arrange the following clinical steps in the optimal sequence to diagnose and manage his suspected acute cellular rejection.

A 28-year-old woman is brought to the emergency department by ambulance due to the sudden onset of difficulty breathing, lip swelling, and a diffuse, intensely pruritic rash. The symptoms began approximately 15 minutes after she received her first dose of intravenous ceftriaxone for uncomplicated pyelonephritis at an urgent care clinic. Her temperature is 37.1°C (98.8°F), blood pressure is 78/45 mmHg, heart rate is 128/min, and respiratory rate is 26/min. Physical examination reveals stridor, diffuse expiratory wheezing, and bilateral angioedema of the face and lips. Which of the following mechanisms of action corresponds to the pharmacotherapy that must be administered immediately to stabilize this patient?

A 3-year-old boy is evaluated in the pediatric genetics clinic due to excessive food-seeking behavior and rapid weight gain. His parents report that he had profound muscle hypotonia and required nasogastric tube feeding during his first month of life. Physical examination demonstrates central adiposity, small hands and feet, and undescended testes. Initial diagnostic testing with fluorescence in situ hybridization (FISH) for a microdeletion on chromosome 15q11-q13 is negative. Subsequent molecular testing with DNA methylation analysis reveals the presence of only maternally methylated alleles at the 15q11-q13 locus. Which of the following molecular mechanisms best explains these diagnostic findings?

A 4-month-old male infant is evaluated in the pediatric neurology clinic for severe generalized hypotonia, a weak cry, and absent deep tendon reflexes. Physical examination reveals prominent tongue fasciculations and paradoxical breathing. Genetic testing confirms a homozygous deletion of the survival motor neuron 1 (SMN1) gene. The medical team recommends initiating therapy with an intrathecal antisense oligonucleotide (ASO) designed to modulate the expression of the paralogous SMN2 gene. Arrange the following molecular and cellular events in the exact chronological order by which this targeted genetic therapy exerts its clinical effect, from initial molecular binding to ultimate cellular rescue.

During a routine health maintenance visit, a 42-year-old male accountant reports a gradual but significant decline in his exercise tolerance over the past two years. Previously an avid runner, he now experiences dyspnea after walking up a single flight of stairs. He is a lifelong non-smoker. His family history is notable for a father who died from cryptogenic liver cirrhosis at age 52. On physical examination, the patient has an increased anteroposterior chest diameter and globally diminished breath sounds. Spirometry reveals a forced expiratory volume in one second to forced vital capacity ratio ($\text{FEV}_1/\text{FVC}$) of $62\%$, and carbon monoxide diffusing capacity (DLCO) is reduced to $45\%$ of the predicted value. Serum laboratory analysis demonstrates an aspartate aminotransferase level of $85$ U/L (reference range: $8-40$ U/L) and an alanine aminotransferase level of $92$ U/L (reference range: $7-56$ U/L). A chest radiograph displays significant hyperlucency at the lung bases. Which of the following best describes the molecular mechanism responsible for this patient's hepatic findings?

A 32-year-old man comes to the clinic due to a painful, swollen boil on his left thigh. Physical examination shows a 3-cm erythematous, fluctuant skin abscess. He undergoes incision and drainage, and a sample is sent for culture. The culture grows Staphylococcus aureus that is resistant to oxacillin and cefazolin. Which of the following is the primary molecular mechanism responsible for this organism's antibiotic resistance?

Parents bring their 5-year-old son to the physician for evaluation of delayed motor milestones and a waddling gait. When rising from the floor, he places his hands on his thighs and pushes upwards to stand. Physical examination demonstrates prominent proximal muscle weakness and bilaterally enlarged, firm calf muscles. The physician strongly suspects an underlying X-linked recessive neuromuscular disorder. Arrange the following diagnostic interventions in the most appropriate sequential order to definitively establish the diagnosis according to current guidelines.

A 32-year-old man visits the primary care clinic to establish care and discuss management of recently diagnosed essential hypertension. He reports a lifelong history of episodic, unprovoked swelling of his lips, tongue, and extremities. These episodes typically last 48 to 72 hours and are not associated with urticaria or pruritus. His father and paternal grandfather experienced similar symptoms throughout their lives. Recent laboratory testing confirms a deficiency of C1 esterase inhibitor. The patient's blood pressure today is 152/94 mm Hg. The physician decides to initiate antihypertensive therapy. Which of the following medications is strictly contraindicated in this patient?