First Aid Step 1 Rapid Review: Disease Mechanisms, Clinical Associations, and High-Yield Concepts

Comprehensive review flashcards covering pathophysiology mechanisms, diagnostic interpretations, clinical vignettes, formulas, and pharmacology from First Aid Step 1 Rapid Review.

What is the underlying pathophysiological mechanism of Lynch syndrome (HNPCC)?

Mutation in MMR genes (MLH1, MSH2) leading to failure of DNA mismatch repair during the S phase, resulting in microsatellite instability and an increased risk of colorectal cancer ($\approx 80\%$).

What is the genetic and biochemical defect in Osteogenesis imperfecta?

Mutation in the COL1A1 and COL1A2 genes resulting in a Type 1 collagen defect and an inability to form triple helices.

What is the molecular mechanism of Menkes disease?

Defective ATP7A protein causing impaired copper absorption and transport, leading to decreased lysyl oxidase activity and decreased collagen cross-linking.

What is the genetic mechanism of Prader-Willi syndrome?

Imprinting or maternal uniparental disomy ($25\%$) resulting in deletion or mutated expression of the paternal allele on chromosome 15.

What is the pathophysiological mechanism of Cystic fibrosis?

$\Delta\text{F508}$ CFTR deletion on chromosome 7 leading to an impaired ATP-gated $\text{Cl}^-$ channel, which causes decreased $\text{Cl}^-$ secretion in the lungs and GI tract and decreased $\text{Cl}^-$ reabsorption in sweat glands.

How does Becker muscular dystrophy differ mechanically and clinically from Duchenne muscular dystrophy?

Becker muscular dystrophy is caused by a non-frameshift deletion of the dystrophin gene resulting in a partially functional protein, making it clinically less severe than Duchenne (which is caused by frameshift or nonsense mutations).

What is the genetic defect and molecular consequence in Fragile X syndrome?

CGG trinucleotide repeat expansion in the FMR1 gene leading to hypermethylation of cytosine residues and decreased gene expression.

Why does glucose infusion worsen Wernicke encephalopathy in patients with alcohol use disorder?

Thiamine deficiency impairs glucose breakdown; administering glucose without thiamine accelerates cellular ATP depletion.

What is the pathophysiological mechanism of edema and fatty liver in Kwashiorkor?

Severe protein malnutrition leads to decreased plasma oncotic pressure (causing edema) and decreased apolipoprotein synthesis (causing hepatic steatosis/fatty changes).

What is the biochemical defect and consequence in Hereditary fructose intolerance?

Aldolase B deficiency causes accumulation of fructose-1-phosphate, resulting in depletion of intracellular available phosphate and subsequent inhibition of glycogenolysis and gluconeogenesis.

What is the enzyme deficiency in Classic galactosemia, and why do cataracts develop?

Galactose-1-phosphate uridyltransferase deficiency; cataracts result from the accumulation of toxic galactitol in the lens of the eye.

Why do the lens, retina, kidneys, and Schwann cells develop osmotic damage in diabetes mellitus?

Excess glucose is converted to sorbitol via aldose reductase; these specific tissues lack sorbitol dehydrogenase to convert sorbitol into fructose, causing intracellular sorbitol accumulation and osmotic injury.

What is the molecular mechanism underlying Hereditary angioedema?

C1 inhibitor deficiency leads to unregulated activation of kallikrein, which causes increased levels of bradykinin.

What are the immediate and late phase mechanisms of Type I hypersensitivity?

Immediate phase (minutes): antigen crosslinks IgE on mast cells, triggering degranulation and release of histamine, tryptase, and leukotrienes. Late phase (hours): mast cells secrete chemokines that recruit inflammatory cells and mediators, causing tissue damage.

What is the immunological mechanism of Type III hypersensitivity?

Antigen-antibody (immune) complexes deposit in tissue and activate complement, which attracts neutrophils that release lysosomal enzymes.

What is the mechanism of an acute hemolytic transfusion reaction?

A Type II hypersensitivity reaction in which preformed recipient antibodies destroy donor RBCs (typically against ABO antigens).

What embryonic structure fails to develop in DiGeorge syndrome, and what is the underlying genetic defect?

A 22q11 microdeletion causes failure of development of the 3rd and 4th branchial (pharyngeal) pouches, leading to an absent thymus and absent parathyroid glands.

What is the molecular defect in Leukocyte adhesion deficiency (type 1)?

Defect in the LFA-1 integrin (CD18), impairing phagocyte migration, adhesion, and chemotaxis.

What enzyme defect causes Chronic granulomatous disease (CGD)?

Deficiency in NADPH oxidase, resulting in decreased reactive oxygen species (ROS) and diminished respiratory burst in neutrophils, which increases susceptibility to catalase-positive organisms.

What is the immunological mechanism of Graft-versus-host disease (GVHD)?

A Type IV hypersensitivity reaction caused by HLA mismatch where viable donor T cells attack host cells.

What is the toxin mechanism in Hemolytic-uremic syndrome (HUS)?

Shiga or Shiga-like toxins inactivate the 60S ribosomal subunit, halting protein synthesis and triggering cytokine release.

What is the mechanism of action of botulinum toxin in botulism?

The toxin is a protease that cleaves SNARE proteins, inhibiting acetylcholine (ACh) release at the neuromuscular junction.

How do superantigens in Toxic shock syndrome (TSST-1) and scarlet fever trigger massive cytokine release?

TSST-1 and erythrogenic exotoxin A cross-link the $\beta$ region of the T-cell receptor (TCR) to MHC class II molecules on antigen-presenting cells outside the antigen-binding groove, causing massive release of IL-1, IL-2, IFN-$\gamma$, and TNF-$\alpha$.

What mechanism enables Staphylococcus epidermidis to adhere to and infect prosthetic medical devices?

Production of a protective biofilm.

What is the pathogenic mechanism of Clostridioides difficile in pseudomembranous colitis?

Toxins A and B damage enterocytes, resulting in mucosal inflammation and watery diarrhea.

What virulence factors contribute to the pathogenesis of Mycobacterium tuberculosis?

Cord factor (activates macrophages, promotes granuloma formation, and induces TNF-$\alpha$ release) and sulfatides (surface glycolipids that inhibit phagolysosomal fusion).

What immune response characterizes lepromatous leprosy compared to tuberculoid leprosy?

Lepromatous leprosy is characterized by a predominantly Th2 immune response that fails to contain the infection and can be lethal (unlike the Th1-dominated, milder tuberculoid leprosy).

What virulence factor allows uropathogenic E. coli to cause cystitis and pyelonephritis?

Fimbriae (P pili), which mediate attachment to uroepithelium.

Why are Chlamydiae inherently resistant to $\beta$-lactam antibiotics?

They lack classic peptidoglycan due to markedly reduced muramic acid content in their cell walls.

What is the molecular defect and clinical consequence of Bernard-Soulier syndrome?

Deficiency of glycoprotein GpIb, leading to impaired platelet adhesion to von Willebrand factor (vWF).

What is the pathophysiology of Thrombotic thrombocytopenic purpura (TTP)?

Decreased ADAMTS13 (a vWF metalloprotease) leads to impaired cleavage of ultra-large vWF multimers, promoting platelet adhesion, microthrombi formation, and thrombocytopenia.

What is the specific mutation and mechanism in Factor V Leiden?

Arg506Gln point mutation in factor V, rendering it resistant to degradation and inactivation by activated protein C, thereby creating a hypercoagulable state.

Why does warfarin administration occasionally cause skin and tissue necrosis?

Protein C has a much shorter half-life than other vitamin K–dependent clotting factors; initial administration leads to a rapid decline in protein C before clotting factors are depleted, triggering transient hypercoagulation and microthrombosis.

What mechanisms and clinical signs are associated with radial nerve injury ('Saturday night palsy')?

Caused by axillary compression (crutch use), midshaft humerus fractures, or repetitive pronation/supination; presents with wrist drop and decreased grip strength.

How do proximal and distal ulnar nerve injuries differ clinically?

Proximal lesion (fracture of medial epicondyle): radial deviation of wrist on flexion. Distal lesion (fracture of hook of hamate): ulnar claw on digital extension and sensory loss over ulnar digits.

What mechanical traction causes Klumpke palsy, and which nerve roots are affected?

Traction or tearing of the C8-T1 nerve roots, occurring in infants during delivery by pulling on an abducted arm or in adults grabbing a tree branch while falling.

What injury leads to common peroneal nerve palsy, and what gait abnormality results?

Blunt trauma to the lateral leg or a fibular neck fracture, presenting as foot drop and a characteristic 'steppage gait'.

What etiology and clinical manifestations characterize pudendal nerve injury?

Caused by prolonged cycling or horseback riding; presents with decreased sensation in the perineal and genital regions with possible fecal or urinary incontinence.

What is the mechanism of Slipped capital femoral epiphysis (SCFE)?

Increased axial loading/force on the femoral head in an obese adolescent causes the femoral epiphysis to displace relative to the femoral neck.

What cellular imbalance drives the reduction in bone mass in Osteoporosis?

Increased osteoclast activity relative to osteoblast activity, secondary to estrogen depletion, advanced age, or long-term medication use (e.g., corticosteroids).

What is the bone remodeling abnormality in Osteitis deformans (Paget disease of bone)?

Marked increase in osteoclastic activity followed by a disordered surge in osteoblastic activity, producing poor quality, mechanically weak, fracture-prone mosaic bone.

What is the pathophysiological mechanism of Rheumatoid arthritis joint destruction?

Autoimmune inflammation resulting in proliferative pannus formation that progressively erodes articular cartilage and subchondral bone.

What hypersensitivity reactions mediate the pathophysiology of Systemic lupus erythematosus (SLE)?

Predominantly a Type III hypersensitivity reaction characterized by deficient clearance of circulating immune complexes; hematologic cytopenias are mediated via Type II hypersensitivity.

What is the autoimmune target in Myasthenia gravis?

Autoantibodies directed against postsynaptic nicotinic acetylcholine (ACh) receptors at the neuromuscular junction.

What is the cellular basis of Albinism?

Normal melanocyte count accompanied by decreased melanin production due to deficient tyrosinase activity.

What gene mutation and skin defect are central to the pathogenesis of Atopic dermatitis?

Loss-of-function mutations in the filaggrin (FLG) gene causing epidermal barrier dysfunction, coupled with immune dysregulation and an altered skin microbiome.

What antibody target and histologic plane define Pemphigus vulgaris?

Type II hypersensitivity with IgG autoantibodies targeting desmoglein 1 and 3 in desmosomes, causing acantholysis and intraepidermal separation above the basal layer (positive Nikolsky sign).

What developmental defect causes Spina bifida (occulta, meningocele, myelomeningocele)?

Failure of the caudal neuropore to fuse properly by the 4th week of embryological development.

What developmental failure characterizes Holoprosencephaly, and what signaling pathway is implicated?

Failure of the prosencephalon (forebrain) to divide into two distinct cerebral hemispheres during weeks 3–4 of development; associated with Sonic Hedgehog (SHH) gene mutations.

What structural herniation defines a Chiari I malformation?

Downward displacement and herniation of the cerebellar tonsils through the foramen magnum.

What cerebellar abnormality characterizes Dandy-Walker malformation?

Agenesis of the cerebellar vermis accompanied by cystic dilatation of the 4th ventricle filling an expanded posterior fossa.

What anatomical lesion causes Gerstmann syndrome, and what are its four hallmark clinical features?

Lesion in the dominant parietal cortex; presents with agraphia, acalculia, finger agnosia, and left-right disorientation.

What anatomical lesion causes Klüver-Bucy syndrome, and what viral encephalitis classically precipitates it?

Bilateral lesions of the amygdala; classically caused by HSV-1 encephalitis, producing hyperorality, hypersexuality, and hyperphagia.

What causes the brainstem pathology in Locked-in syndrome, and what movement is characteristically preserved?

Infarction of the paramedian ventral pons secondary to basilar artery thrombosis; vertical eye movements and blinking are preserved while horizontal movements are lost.

What vascular occlusion causes Lateral medullary (Wallenberg) syndrome?

Thrombosis or occlusion of the posterior inferior cerebellar artery (PICA).

Where does neonatal intraventricular hemorrhage typically originate in premature infants?

In the germinal matrix, a delicate and highly vascularized subventricular zone with poor glial fiber support and vulnerable autoregulation.

What vessels rupture to produce a subdural hematoma?

Bridging cortical veins.

What is the underlying defect in Proximal renal tubular acidosis (type 2)?

Defective reabsorption of $\text{HCO}_3^-$ in the proximal convoluted tubule, causing increased urinary excretion of bicarbonate and systemic metabolic acidosis.

What is the primary mechanism producing hematuria and oliguria in Nephritic syndrome?

Immune-mediated glomerular inflammation damages the glomerular basement membrane (causing dysmorphic RBC leakage) and depresses GFR (producing oliguria and azotemia).

What is the genetic mode of inheritance and structural consequence of Alport syndrome?

X-linked dominant mutation in Type IV collagen causing irregular thinning, thickening, and longitudinal splitting of the glomerular basement membrane.

What is the primary mechanism of Urgency incontinence?

Detrusor smooth muscle overactivity leading to an uninhibited contraction and immediate urge to void.

What hemodynamic change drives Prerenal azotemia?

Decreased renal blood flow (RBF) resulting in a drop in GFR, triggering compensatory avid reabsorption of sodium, water, and urea.

What vascular maladaptation initiates Preeclampsia?

Abnormal remodeling of maternal spiral arteries by endovascular trophoblasts, causing placental ischemia, generalized endothelial dysfunction, and vasoconstriction.

How does insulin resistance drive androgen excess in Polycystic ovary syndrome (PCOS)?

Hyperinsulinemia alters hypothalamic gonadotropin feedback (elevating the LH:FSH ratio) and stimulates ovarian theca cell androgen production, inhibiting follicular rupture.

What venous structure becomes engorged in a varicocele?

The pampiniform venous plexus, dilating secondary to elevated venous pressure (classically described as a 'bag of worms').

Why do patients with sarcoidosis develop hypercalcemia?

Noncaseating granulomas contain activated macrophages that independently express $1\alpha$-hydroxylase, driving excess production of active vitamin D ($1,25\text{-(OH)}_2\text{D}_3$).

A patient presents with situs inversus, chronic sinusitis, bronchiectasis, and infertility. What is the diagnosis?

Primary ciliary dyskinesia (Kartagener syndrome).

A child presents with hypermobile joints, hyperextensible skin, and easy bruising. What collagen types are typically involved?

Type V collagen (classical Ehlers-Danlos syndrome) or Type III collagen (vascular subtype).

How does the lens dislocation of Homocystinuria differ from that of Marfan syndrome?

Homocystinuria causes downward and inward (inferomedial) subluxation of the lens, whereas Marfan syndrome causes upward and outward (superotemporal) subluxation.

A young boy requires his hands and arms to push himself up from a squatting posture. What is this sign called, and what disorder does it indicate?

Gowers sign, indicating Duchenne muscular dystrophy (or proximal lower extremity weakness).

What genetic chromosomal anomaly is associated with a single transverse palmar crease, flat facies, and epicanthal folds?

Down syndrome (Trisomy 21).

An infant presents with microcephaly, cleft lip/palate, holoprosencephaly, polydactyly, and cutis aplasia. What is the diagnosis?

Patau syndrome (Trisomy 13).

What is the classic clinical triad of Wernicke encephalopathy?

Confusion, ophthalmoplegia/nystagmus, and ataxia.

What dietary deficiency causes the '3 Ds' (dermatitis, diarrhea, dementia)?

Niacin (Vitamin B3) deficiency (Pellagra).

Which biochemical pathway abnormality causes subacute combined degeneration, paresthesias, and megaloblastic anemia?

Cobalamin (Vitamin B12) deficiency.

What condition presents with bowlegs in a young child, bone tenderness, and muscle weakness?

Rickets (secondary to Vitamin D deficiency).

A child has intellectual disability, fair/hypopigmented skin, eczema, and a musty body odor. What condition is present?

Phenylketonuria (PKU), caused by phenylalanine hydroxylase or tetrahydrobiopterin ($\text{BH}_4$) deficiency.

A patient on chronic steroid therapy abruptly stops medication and develops vomiting, abdominal pain, shock, and altered mental status. What is the diagnosis?

Acute adrenal insufficiency (adrenal crisis).

What endocrine triad comprises Multiple Endocrine Neoplasia type 2A (MEN2A)?

Medullary thyroid carcinoma, pheochromocytoma, and parathyroid hyperplasia (due to autosomal dominant RET proto-oncogene mutation).

Cutaneous flushing, watery diarrhea, bronchospasm, and a right-sided cardiac valve murmur suggest what condition?

Carcinoid syndrome (associated with metastatic carcinoid tumors; marked by elevated urinary 5-HIAA).

What syndrome presents with longitudinal, partial-thickness esophageal mucosal lacerations and hematemesis after retching?

Mallory-Weiss syndrome.

What systemic disease caused by Tropheryma whipplei presents with arthralgias, diarrhea, and neurologic/cardiac symptoms?

Whipple disease.

What is the clinical significance of severe tenderness at McBurney point upon deep palpation?

Positive McBurney sign, pointing to acute appendicitis.

Thousands of adenomatous colonic polyps accompanied by osteomas and supernumerary impacted teeth characterize which syndrome?

Gardner syndrome (a subtype of familial adenomatous polyposis).

What copper-associated ophthalmologic sign is characteristic of Wilson disease?

Kayser-Fleischer rings (golden-brown copper deposition in Descemet's membrane around the peripheral cornea).

What does a bluish-purple pigmented line along the gingival margin indicate?

Burton line, seen in chronic lead poisoning.

A patient notices dark red/pink urine upon waking up, along with pancytopenia and deep venous thrombosis. What is the diagnosis?

Paroxysmal nocturnal hemoglobinuria (PNH).

Purple, violaceous cutaneous or oral mucosal plaques in an HIV-positive patient are caused by which infectious agent?

Human herpesvirus 8 (HHV-8), causing Kaposi sarcoma.

What does dorsiflexion of the great toe with fanning of the lesser toes upon lateral plantar stimulation indicate?

Positive Babinski sign, indicating an upper motor neuron (UMN) lesion.

What autosomal dominant neurodegenerative disease presents with progressive chorea, dementia, and caudate atrophy?

Huntington disease (CAG trinucleotide expansion).

A 30-year-old female presents with relapsing and remitting episodes of optic neuritis, internuclear ophthalmoplegia, and scanning speech. What is the diagnosis?

Multiple sclerosis.

What genetic neurocutaneous condition is characterized by multiple café-au-lait macules, Lisch nodules, and neurofibromas?

Neurofibromatosis type I.

A facial port-wine stain (nevus flammeus) in the distribution of the ophthalmic branch of the trigeminal nerve is associated with which syndrome?

Sturge-Weber syndrome.

An adolescent presents with progressive ataxia, nystagmus, hammer toes, and hypertrophic cardiomyopathy. What is the diagnosis?

Friedreich ataxia (GAA repeat on chromosome 9).

What clinical constellation defines Ménière disease?

Episodic vertigo, sensorineural hearing loss, and tinnitus with a feeling of ear fullness (caused by endolymphatic hydrops).

What classic pathognomonic cell features bilobed nuclei with prominent 'owl-eye' inclusion-like nucleoli in Hodgkin lymphoma?

Reed-Sternberg cells.

What skeletal radiographic sign and peripheral blood smear feature are classically seen in Multiple myeloma?

Punched-out osteolytic bone lesions on plain radiographs and Rouleaux formation of red blood cells on peripheral smear.

Smudge cells (fragile leukocytes ruptured during slide preparation) on a peripheral blood smear are diagnostic of which leukemia?

Chronic lymphocytic leukemia (CLL).

A lytic bone lesion on x-ray with an expanded 'soap bubble' appearance in the epiphysis/metaphysis of the distal femur or proximal tibia represents what tumor?

Giant cell tumor of bone.

What bone malignancy produces a concentric 'onion skin' periosteal reaction on diaphyseal plain films?

Ewing sarcoma.