MFM 1-4
This enzyme extends chromosome ends by using its own RNA template, preventing progressive shortening during DNA replication.
What is telomerase?
(Telomerase uses its own RNA template to add repetitive DNA sequences to chromosome ends, preventing progressive telomere shortening)
A neuron contains normal microtubules but cannot transport newly synthesized secretory vesicles from its cell body to its axon terminal. Which protein is most likely dysfunctional?
What is kinesin?
(Kinesin moves cargo toward the microtubule’s plus end, transporting vesicles from the neuronal cell body toward the axon terminal)
A patient can abduct the arm once it is lifted beyond 15°, but cannot initiate abduction from the anatomical position. Which muscle is most likely injured?
What is the supraspinatus?
(Supraspinatus initiates the first 15° of abduction; the deltoid primarily continues abduction from 15° to 120°)
A mutation disrupts production of sheet-forming type IV collagen. Which structure would be most directly weakened?
What is the basal lamina?
(Type IV collagen forms sheets and is a major structural component of basal and external laminae)
A mass compresses the structures passing through the quadrangular space. Which combination of findings is most likely?
Weak arm abduction from 15° to 120° and numbness over the lateral shoulder.
(The axillary nerve passes through the quadrangular space and supplies the deltoid and skin over the lateral shoulder)
A mutation allows eukaryotic cells to produce RNA–DNA primers normally, but the cells accumulate short, incompletely extended Okazaki fragments. Leading-strand and mitochondrial DNA synthesis remain intact. Which DNA polymerase is most likely defective?
What is DNA polymerase δ?
(Polymerase α makes the initial RNA–DNA primer, while polymerase δ primarily extends Okazaki fragments on the lagging strand)
A child has recurrent bacterial infections and persistently elevated circulating neutrophils. Testing shows that the neutrophils can roll along vascular endothelium but cannot establish firm adhesion. Which interaction is defective?
What is the integrin–ICAM interaction?
(Selectins allow neutrophils to roll, but leukocyte integrins must bind endothelial ICAMs for firm adhesion before transmigration)
Following axillary lymph-node removal, a patient has difficulty pushing against a wall. The medial border of the scapula projects posteriorly during the attempt. Which nerve was most likely injured?
What is the long thoracic nerve?
(Long thoracic nerve injury paralyzes serratus anterior, producing a winged scapula)
A toxin selectively destroys the rough endoplasmic reticulum and free ribosomes within a neuron. Which change would most likely be observed?
What is loss of Nissl substance with reduced protein synthesis?
(Nissl bodies are aggregates of rough ER and free ribosomes, so they reflect neuronal protein synthesis)
A lymphoid organ loses the delicate branching framework that normally supports its numerous free cells. Analysis shows a deficiency of type III collagen. Which fibers are affected?
What are reticular fibers?
(Reticular fibers are composed primarily of type III collagen and form delicate supporting networks)
A mutation prevents phosphorylation of the carboxy-terminal domain (CTD) of RNA polymerase II. The polymerase can still assemble with transcription factors at the promoter. Which finding would most likely result?
RNA polymerase II cannot efficiently leave the promoter and transition into elongation.
(CTD stands for the carboxy-terminal domain, which is the tail of eukaryotic RNA polymerase II. When the CTD is phosphorylated, RNA polymerase II can leave the promoter and transition from initiation into elongation. It also acts as a docking site for RNA-processing proteins.)
A mutation locks Gαq in its inactive, GDP-bound state. Following receptor stimulation, which intracellular response would be reduced?
Release of Ca²⁺ from the endoplasmic reticulum.
(Gαq activates phospholipase C, producing IP₃ and DAG. IP₃ triggers Ca²⁺ release from the ER, so inactive Gαq reduces this response)
A patient loses shoulder abduction from 15° to 120° and develops wrist drop following an axillary injury. Which portion of the brachial plexus was most likely damaged?
What is the posterior cord?
(The posterior cord gives rise to both the axillary nerve, which supplies the deltoid, and the radial nerve, which supplies the wrist extensors)
A genetic defect prevents fibroblasts from performing their principal function. Which tissue change would most likely occur?
Reduced production of collagen, elastin, and ground substance
(Fibroblasts synthesize and maintain collagen, elastin, and components of ground substance)
A medication inhibits nitric oxide synthase in vascular endothelial cells. Which downstream change is expected in adjacent vascular smooth muscle?
What are decreased cGMP and reduced smooth-muscle relaxation?
(Endothelial NO normally activates guanylyl cyclase in smooth muscle, increasing cGMP and causing relaxation)
A patient’s cells can repair uracil produced by cytosine deamination but cannot efficiently remove bulky, helix-distorting thymine dimers after UV exposure. Which repair activity is defective?
What is nucleotide excision repair (NER)?
(NER removes bulky, helix-distorting lesions such as UV-induced thymine dimers. Base excision repair handles small lesions such as uracil from cytosine deamination)
A cytokine binds normally to its receptor and causes receptor dimerization, but a mutation prevents JAK from phosphorylating the receptor’s intracellular tail. Which subsequent event will fail?
What is failure of STAT activation and nuclear translocation?
(Without phosphorylated receptor tyrosines, STAT proteins cannot dock through their SH2 domains)
A mid-shaft humeral fracture injures the radial nerve within the spiral/radial groove. The patient develops wrist drop but retains the ability to extend the elbow. What best explains the preserved elbow extension?
The triceps receives its radial innervation proximal to the fracture.
(The radial nerve supplies the triceps before traveling through the spiral groove, so a lesion at the humeral shaft usually spares elbow extension)
An infant has normally developed neurons within the brain and spinal cord but has abnormal dorsal root ganglia and Schwann cells in the peripheral nervous system. Development of which embryologic cell population was most likely disrupted?
What are neural crest cells?
(The CNS develops primarily from the neural tube, while dorsal root ganglia and Schwann cells develop from neural crest cells)
A patient produces antibodies against dystonin. A skin biopsy shows separation of an intact epithelial sheet from the underlying basal lamina. Which junction has been disrupted?
What is a hemidesmosome?
(Dystonin is a hemidesmosomal component. Hemidesmosomes anchor epithelial cells to the basal lamina)
A drug selectively inhibits the sodium–glucose symporter responsible for glucose reabsorption in the proximal tubule of the kidney. The patient develops increased urinary glucose, while intestinal glucose absorption remains largely intact. Which transporter is inhibited?
What is SGLT2?
(SGLT2 uses the Na⁺ gradient to reabsorb glucose in the kidney’s proximal tubule. SGLT1 primarily supports glucose absorption in the small intestine)
A soluble protein normally resides in the ER but is accidentally transported to the cis-Golgi. A mutation has removed its KDEL sequence. What is the most likely fate of the protein?
What is continued movement through the secretory pathway?
(It will continue through the secretory pathway because it cannot be retrieved)
A patient falls on an outstretched hand. Initial radiographs are inconclusive, but examination reveals marked tenderness in the anatomical snuffbox. Failure to recognize the injury places which bone at greatest risk for avascular necrosis?
What is the scaphoid?
(A FOOSH can fracture the scaphoid, whose limited blood supply places it at risk for avascular necrosis)
A mutation prevents calcium from binding to troponin in skeletal muscle. An action potential still reaches the muscle fiber, and calcium is released normally from the sarcoplasmic reticulum. Why does contraction fail?
What is continued blockage of actin’s myosin-binding sites by tropomyosin?
(Calcium binding to troponin normally shifts tropomyosin away from the binding sites)
A tumor cell has a loss-of-function mutation in APC. Even in the absence of extracellular Wnt, which finding is most likely?
What are β-catenin accumulation and activation of TCF-regulated genes?
(APC normally participates in the complex that targets β-catenin for degradation. Loss of APC allows β-catenin to accumulate, enter the nucleus, and activate transcription)