Week 1
A researcher is studying bacterial genetics and notes that a single nucleotide substitution in a gene encoding an essential enzyme completely abolishes its functional activity in Escherichia coli. Which genomic characteristic explains why bacteria are particularly vulnerable to single gene mutations?
Haploid singular circular genome: bacteria don't have any back up alleles so it makes them more vulnerable
A 14-month-old unvaccinated child is brought to the emergency department with a 2-day history of high fever, irritability, and severe swelling in the neck region. Physical examination reveals an anxious child who is leaning forward, drooling, and exhibiting stridor. Examination of the posterior pharynx is deferred to avoid complete airway obstruction. Which organism is the most likely underlying cause of this presentation?
H.influenzae type B
The symptoms are a classic example of epiglottitis
A 62-year-old male with end-stage renal disease on hemodialysis presents to the emergency department with 2 days of high fever, chills, and productive cough. Physical examination reveals a temperature of 38.9 °C (102 °F), tachypnea, and bronchial breath sounds over the right lower lung field. Chest radiography confirms a dense right lower lobe consolidation. Sputum Gram stain demonstrates lancet-shaped Gram-positive diplococci. The attending physician selects a parenteral beta-lactam antibiotic that is active against Streptococcus pneumoniae and eliminated primarily in bile, requiring no renal dosage adjustment. Which antimicrobial agents was most likely administered?
Ceftriaxone is a third-generation cephalosporin active against S. pneumoniae that undergoes significant biliary elimination. Unlike most other beta-lactams, it requires no dosage adjustment in renal impairment. Cefazolin, cefepime, piperacillin, and imipenem are all eliminated renally.
A 41-year-old man with advanced AIDS has painful perianal herpes lesions that keep spreading despite high-dose acyclovir. Testing shows his HSV strain has a deficient thymidine kinase. What drug is most appropriate?
Foscarnet is a pyrophosphate derivative that needs no kinase activation and is approved for acyclovir-resistant HSV.
Acyclovir resistance usually comes from altered or deficient viral thymidine kinase, mostly in immunocompromised patients, with cross-resistance to related TK-activated drugs (valacyclovir, famciclovir, penciclovir).
A 5-month-old infant is brought to the emergency room with a 2-day history of constipation, weak cry, poor feeding, ptosis, loss of head control ('floppy baby syndrome'), and generalized flaccid paralysis. The parents report feeding the infant raw unpasteurized honey 3 days prior. Which mechanism explains why infants acquire botulism from honey ingestion, whereas healthy adults do not acquire botulism from ingesting the same spores?
Infant gastrointestinal tract lacks established normal microbiota, permitting spore germination, bacterial colonization, and in vivo toxin production.
A patient presents with symptoms of an atypical lower respiratory tract infection. Biochemical and structural analysis of the isolated causative bacterium reveals an absence of a peptidoglycan cell wall, but its cytoplasmic membrane unexpectedly contains sterols. Which organism is most likely responsible for this infection?
Mycoplasma pneumoniae
A 26-year-old female presents to the outpatient clinic complaining of dysuria and purulent cervical discharge. Cervical Gram stain demonstrates numerous polymorphonuclear leukocytes but no intracellular or extracellular bacterial organisms. A culture on routine blood and MacConkey agar yields no growth after 48 hours. Which feature of the suspected pathogen explains its inability to grow on standard nutrient agar?
Chlamydia species are obligate intracellular organism who require eukaryotic cell ATP and nutrients
A 2-day-old neonate born prematurely at 30 weeks gestation develops abdominal distension, severe hypothermia, cyanosis, and progressive cardiovascular collapse. Medical history reveals that the mother was treated with an old broad-spectrum antibiotic for a suspected tropical bacterial infection shortly before delivery. Physical exam shows ashen-gray discoloration of the skin and flaccid extremities. Laboratory studies reveal severe metabolic acidosis and hyperbilirubinemia. Which metabolic deficiencies in the neonate accounts for this drug toxicity and what drug caused this reaction?
Chloramphenicol toxicity in neonates causes Gray Baby Syndrome. Premature infants lack adequate hepatic UDP-glucuronosyltransferase activity and have immature renal clearance, leading to accumulation of toxic drug levels, mitochondrial ribosome inhibition, hypothermia, gray cyanosis, and shock.
A 60-year-old man with acute myelogenous leukemia and prolonged neutropenia develops fever, pleuritic chest pain, and hemoptysis. A chest computed tomography (CT) scan shows a dense pulmonary nodule surrounded by a halo of hemorrhage. Bronchoalveolar lavage reveals acutely branching septate hyphae. Which host cell parameter normally serves as the primary defense against this fungal pathogen's hyphal form?
Neutrophil reactive oxygen species
Aspergillus fumigatus invasion is primarily cleared by neutrophils, which destroy hyphae via reactive oxygen species (ROS) and oxidative bursts. Alveolar macrophages phagocytose inhaled conidia (spores), but once hyphae form, neutrophils are the main line of defense. Prolonged neutropenia is the major risk factor for invasive aspergillosis.
A 23-year-old female presents with fever, right wrist pain, and painful skin lesions on her extremities. Physical examination reveals tenosynovitis of the right wrist and flexor tendons, along with scattered necrotic pustules on an erythematous base over her fingers and toes. Arthrocentesis of her right wrist yields purulent fluid. Blood cultures grow oxidase-positive Gram-negative diplococci that ferment glucose only. Which virulence factor undergoes high-frequency antigenic variation to evade host antibody-mediated clearance during infection?
Neisseria gonorrhoeae causes Disseminated Gonococcal Infection (DGI), presenting with the classic triad of tenosynovitis, dermatitis (pustules on a hemorrhagic base), and purulent arthritis. N. gonorrhoeae evades host immune memory via high-frequency antigenic variation of Type IV Pili driven by pilS (silent) to pilE (expressed) gene conversion.
A 34-year-old textile worker presents to the urgent care clinic with a painless cutaneous ulcer on his right forearm. The patient states that the lesion began as a small papule a few days ago after he was handling imported raw animal hides.Physical examination reveals a central black eschar surrounded by striking non-pitting local edema. Gram stain of the fluid exuded from the lesion demonstrates large, square-ended Gram-positive rods in long chains. What is the mechanism of action of the toxin component responsible for the soft tissue swelling in this patient?
Adenylate cyclase that increases intracellular cyclic AMP (cAMP) levels.
Bacillus anthracis causes cutaneous anthrax characterized by a painless ulcer with a central black eschar and severe local edema. Anthrax toxin consists of Protective Antigen (B subunit), Edema Factor (EF), and Lethal Factor (LF). Edema factor is an adenylate cyclase that increases intracellular cAMP, causing fluid exudation and edema.
A 38-year-old male with severe 3rd-degree burn wounds over 40% of his body develops septic shock. Wound dressings display a dark blue-green exudate with a characteristic sweet grape-like odor. Which organism is this and which virulence pigment produced by this pathogen generates toxic reactive oxygen species (superoxide and hydrogen peroxide) that damage host tissue and cilia?
Pseudomonas aeruginosa and pyocyanin: Pseudomonas aeruginosa produces pyocyanin (blue-green pigment) which catalyzes the reduction of oxygen to toxic reactive oxygen species (superoxide, H2O2), damaging host epithelium, cilia, and neutrophils. Pyoverdin is a yellow-green siderophore that sequesters iron.
A 42-year-old male spelunker (cave explorer) presents with hydrophobia, painful pharyngeal spasms when attempting to drink water, agitation, and excessive salivation 6 weeks after sustaining a bat bite. Despite intensive supportive care, his condition deteriorates. Brain autopsy reveals eosinophilic cytoplasmic inclusion bodies within hippocampal and cerebellar Purkinje neurons (Negri bodies). What virus is this and how does it travel from the wound to the CNS?
Rabies virus (Rhabdoviridae) glycoprotein binds acetylcholine receptors at the neuromuscular junction. The virus ascends to the central nervous system via retrograde axonal transport along peripheral motor nerves mediated by dynein motor complexes.
A 29-year-old zoologist returning from a field expedition in East Africa presents with fever, posterior cervical lymphadenopathy (Winterbottom sign), and daytime somnolence. Blood smear demonstrates motile, flagellated trypomastigotes. Which vector transmitted this infection?
Glossina fly (Tsetse fly)
African sleeping sickness (Trypanosoma brucei rhodesiense / gambiense) is transmitted by the bite of the Glossina fly (tsetse fly). Clinical signs include Winterbottom sign (cervical lymphadenopathy) and progressive CNS involvement (somnolence).
A 22-year-old female presents with nasal congestion, rhinorrhea, sneezing, and sore throat without fever. Viral culture isolates Rhinovirus. The resident physician notes that unlike enteroviruses (such as poliovirus or coxsackievirus), rhinoviruses cause localized upper respiratory tract infections and cannot cause gastrointestinal infections because they are destroyed by stomach acid. Which cellular surface molecules serves as the primary attachment receptor for the majority of human rhinoviruses?
The vast majority (major group) of human rhinoviruses bind to Intercellular Adhesion Molecule 1 (ICAM-1 / CD54) expressed on respiratory epithelial cells. Rhinoviruses are acid-labile (inactivated at pH < 5) and grow optimally at 33 °C (nasal mucosa temperature).
A 31-year-old male presents to the emergency department after stepping on an old, rusty nail while walking barefoot near a farm. Five days later, he experiences jaw stiffness, difficulty opening his mouth, and painful involuntary facial muscle spasms giving him a constant grin. Laboratory analysis shows that the causative organism is an obligate anaerobic, spore-forming Gram-positive rod. Where in the human body does the active exotoxin exert its primary pathophysiologic effect?
Spinal cord synapses by blocking the release of glycine and GABA.
Clostridium tetani produces tetanospasmin, which travels retrograde intra-axonally to the CNS and binds ganglioside receptors. It blocks the release of inhibitory neurotransmitters (glycine and GABA) at spinal synapses, causing uninhibited muscle firing and spastic paralysis (trismus, risus sardonicus, opisthotonos).
A 52-year-old male receiving infliximab therapy for severe rheumatoid arthritis presents with cough, hemoptysis, fever, and a 15-lb weight loss over 2 months. Chest CT reveals a cavitary lesion in the apical segment of the right upper lobe. Acid-fast staining of bronchoalveolar lavage fluid reveals bacilli growing in tight, serpentine cords in vitro. Which of the following surface molecules is directly responsible for this characteristic microscopic morphology and virulently induces granulomatous inflammation?
Trehalose 6,6'-dimycolate (Cord Factor): A cell wall glycolipid of M. tuberculosis that causes bacilli to align in serpentine cords in culture. Cord factor inhibits phagosome-lysosome fusion, disrupts mitochondrial respiration, and triggers host TNF-alpha secretion driving granulomatous inflammation.
A 28-year-old male presents with 2 weeks of fatigue, low-grade fever, dark urine, and right upper quadrant abdominal tenderness following a needle-stick injury 3 months ago. Serum ALT is 850 U/L and AST is 720 U/L. Hepatitis B serologic panel reveals: HBsAg negative, Anti-HBs negative, Anti-HBc IgM positive, Total Anti-HBc positive, HBeAg negative. The patient is diagnosed with acute Hepatitis B during the 'window period'. Which diagnostic markers is uniquely positive and diagnostic of acute HBV infection during the window period?
The 'window period' in acute HBV occurs when HBsAg has cleared from serum but Anti-HBs antibodies have not yet risen to detectable levels. Anti-HBc IgM is the sole positive serologic marker of acute HBV infection during this window.
A 38-year-old AIDS patient (CD4<50/μLCD4<50/μL) presents with progressive vision loss in his right eye.Ophthalmologic exam reveals fluffy white retinal lesions with hemorrhages ("pizza-pie" retinopathy). Which antiviral agent is first-line treatment for this patient's condition?
Ganciclovir
Cytomegalovirus (CMV) retinitis is a common opportunistic infection in advanced HIV/AIDS (CD4<50/μLCD4<50/μL). Standard first-line therapy is intravenous ganciclovir (or oral valganciclovir).
A 28-year-old woman with a history of intravenous drug use presents with high fever and a new holosystolic murmur best heard at the lower left sternal border that increases with inspiration. Blood cultures grow Gram-positive cocci in clusters that are catalase-positive and coagulase-positive. What is the most appropriate initial intravenous antibiotic therapy?
Vancomycin
The patient has acute tricuspid valve endocarditis (holosystolic murmur increasing with inspiration) caused by Staphylococcus aureus (Gram-positive cocci in clusters, catalase/coagulase positive). Empirical treatment for acute bacterial endocarditis in IV drug users must cover methicillin-resistant S. aureus (MRSA), making intravenous vancomycin the drug of choice.
A 62-year-old male with a 40-pack-year smoking history presents to the emergency department with a 3-day history of high fever, productive cough, and progressive shortness of breath. Sputum Gram stain reveals abundant polymorphonuclear leukocytes but no predominant bacterial organisms. Microscopic and culture analysis are requested. Based on the pathogenesis and environmental resilience of the suspected organism, where does it primarily multiply in its natural water environment prior to human exposure?
Inside free-living amoebae and protozoa
Legionella pneumophilia's natural reservoir includes ponds, lakes, and engineers water systems where it multiplies intracellular within free living amoebae and protozoa.
A 59-year-old male kidney transplant recipient on tacrolimus and prednisone presents with fever, productive cough, and right-sided chest pain. Chest radiograph shows a nodular infiltrate with central cavitation in the right middle lobe. Modified Kinyoun acid-fast stain of sputum demonstrates weakly acid-fast, beaded, branching filamentous bacteria. Brain MRI reveals a solitary ring-enhancing lesion in the left frontal lobe. Which of the following biochemical cell wall features accounts for the weak acid-fast staining property of this organism?
Nocardia species possess shorter, medium-chain mycolic acids (40–60 carbons) compared to Mycobacteria (60–90 carbons). As a result, Nocardia is weakly/partially acid-fast, requiring a modified decolorizing agent (1% sulfuric acid instead of 3% HCl) to retain carbolfuchsin red dye.
A newborn infant born to an unvaccinated mother presents with congenital heart disease (patent ductus arteriosus), bilateral congenital cataracts, and sensorineural hearing loss. Physical examination shows purpuric skin lesions across the trunk ('blueberry muffin' baby appearance). What is the name of the virus and how is it distinguished from other viruses in its family?
Rubella virus is the sole member of the Rubivirus genus in the Togaviridae family. Unlike Alphaviruses (which are mosquito-borne arboviruses), Rubella is transmitted via respiratory droplets and causes German measles and severe congenital rubella syndrome.
A 54-year-old woman returns from East Africa and is brought to the hospital unconscious with pulmonary edema, kidney failure and a high parasitemia of P. falciparum ring forms. Which is the preferred initial treatment?
Artemisinin derivatives are first line for multidrug-resistant P. falciparum, and IV artesunate is preferred for severe malaria, followed by an oral regimen (e.g., artemether–lumefantrine) to complete therapy.
A 35-year-old man with HIV presents with a 3-week history of low-grade fever, dry cough, and progressive dyspnea.Arterial blood gas shows significant hypoxemia. A silver stain of a bronchoalveolar lavage fluid specimen demonstrates cup-shaped cysts within the alveolar space. Which therapeutic agent represents the first-line treatment for this condition?
Trimethoprim-sulfamethoxazole
The clinical presentation and silver stain finding (cup-shaped cysts) are diagnostic for Pneumocystis jirovecii pneumonia (PJP). Trimethoprim-sulfamethoxazole (TMP-SMX) is the first-line treatment and prophylactic agent of choice for PJP.