1500+ medical terms, defined.

A nosocomial or healthcare-associated infection is one acquired in a hospital or healthcare setting, conventionally defined as arising more than forty-eight hours after admission. The commonest are urinary catheter-associated infection,...

Lancet-shaped describes the pointed, oval morphology of Streptococcus pneumoniae, which appears on Gram stain as lancet-shaped gram-positive diplococci. The term also denotes a small sharp blade used for capillary blood sampling.

A diplococcus is a coccus that characteristically occurs in pairs. The medically important examples are the gram-positive lancet-shaped Streptococcus pneumoniae and the gram-negative kidney-bean-shaped Neisseria species and Moraxella cat...

Renshaw cells are inhibitory interneurons in the ventral horn of the spinal cord that receive collaterals from alpha motor neurons and feed back inhibition onto them. They mediate recurrent inhibition, and their loss underlies the spasms...

A toxoid is a bacterial exotoxin that has been chemically inactivated, usually with formaldehyde, so it retains antigenicity but not toxicity. Toxoid vaccines generate neutralizing antibody against the toxin rather than against the organ...

Opisthotonus is a severe extensor spasm in which the back arches, the head retracts, and the heels draw backward. It is a classic sign of generalized tetanus and also occurs in strychnine poisoning, kernicterus, and severe meningitis.

Tetanospasmin is the neurotoxin of Clostridium tetani, a zinc-dependent protease that cleaves SNARE proteins in inhibitory interneurons and blocks release of glycine and GABA. Loss of inhibition produces the spastic paralysis of tetanus.

Nosocomial infections are those acquired during healthcare delivery, arising more than forty-eight hours after admission. Urinary catheters, central lines, ventilators, and surgical wounds account for most of them, and prevention centres...

Vinca alkaloids, vincristine and vinblastine, are antimitotic chemotherapy agents derived from the periwinkle plant that bind tubulin and prevent microtubule polymerization, arresting cells in M phase. Vincristine's dose-limiting toxicit...

Anthrax toxin is a tripartite exotoxin of Bacillus anthracis consisting of protective antigen, edema factor, and lethal factor. Protective antigen mediates cell entry, edema factor is an adenylate cyclase causing massive oedema, and leth...

An eschar is an area of dead, blackened, adherent tissue forming a firm crust over a wound. The painless black eschar with surrounding non-pitting oedema is the hallmark of cutaneous anthrax.

Viridans streptococci are alpha-haemolytic, optochin-resistant, bile-insoluble gram-positive cocci that are normal oral flora. They are the leading cause of subacute bacterial endocarditis on previously damaged valves and are central to...

Haemophilus influenzae is a small gram-negative coccobacillus requiring factors V (NAD) and X (haematin) for growth, cultured on chocolate agar. Type b, the encapsulated form, once caused epiglottitis and meningitis in children but is no...

Streptococcus gallolyticus, formerly Streptococcus bovis, is a group D streptococcus that causes bacteraemia and endocarditis. Its critical clinical association is with colorectal carcinoma, so isolation mandates colonoscopy.

Strep viridans is a shorthand for the viridans group streptococci, alpha-haemolytic oral commensals that are optochin-resistant and bile-insoluble. They cause dental caries and subacute infective endocarditis on damaged valves.

Viridans streptococci are a group of alpha-haemolytic, catalase-negative gram-positive cocci that form part of the normal flora of the mouth, gastrointestinal tract, and genital tract. They are distinguished from pneumococcus by optochin...

Streptococcus sanguinis is a viridans group streptococcus of the oral cavity and dental plaque that is a leading cause of subacute infective endocarditis. It adheres to damaged valves using dextran-mediated binding to fibrin-platelet veg...

Streptococcus mitis is a viridans group alpha-haemolytic streptococcus of the oral cavity that causes subacute endocarditis and bacteraemia in neutropenic patients. In profound neutropenia it can produce a severe septic syndrome with ARDS.

Hemihyperplasia is asymmetric overgrowth of one side of the body or a body region, caused by an excess of normal cells rather than by a tumour. It is associated with Beckwith-Wiedemann syndrome and carries an increased risk of embryonal...

Hemihypertrophy is the older term for hemihyperplasia, asymmetric overgrowth of one side of the body. Its main significance is as a marker for Beckwith-Wiedemann syndrome and for increased risk of Wilms tumour and hepatoblastoma.

Short-term variation (STV) is the beat-to-beat fluctuation in fetal heart rate, quantified by computerized cardiotocography in milliseconds. Reduced STV indicates loss of autonomic modulation and is a recognized marker of fetal acidaemia...

Scarlet fever is a diffuse erythematous rash complicating group A streptococcal pharyngitis, caused by erythrogenic (pyrogenic) exotoxin acting as a superantigen. It produces a sandpaper-textured rash, strawberry tongue, and circumoral p...

Pyogenic infections are pus-forming infections, driven by neutrophil recruitment and liquefactive necrosis. They are characteristically caused by encapsulated and extracellular bacteria such as Staphylococcus aureus, Streptococcus pyogen...

Migratory polyarthritis is joint inflammation that moves from one joint to another, with each site improving as the next becomes involved. It is the commonest major manifestation of acute rheumatic fever and also occurs in disseminated g...

SpeB is streptococcal pyrogenic exotoxin B, a cysteine protease of Streptococcus pyogenes that degrades host tissue and contributes to necrotizing fasciitis. Other Spe toxins, notably SpeA and SpeC, act as superantigens causing streptoco...

Streptolysin S is an oxygen-stable haemolysin of Streptococcus pyogenes responsible for the beta-haemolysis seen on the surface of a blood agar plate. It is not antigenic, so it does not provoke a measurable antibody response.

Streptolysin O is an oxygen-labile, antigenic haemolysin of Streptococcus pyogenes that lyses red and white blood cells. Antibodies against it are measured as the ASO titre, used as evidence of recent streptococcal infection.

MSSA is methicillin-susceptible Staphylococcus aureus, meaning the isolate lacks the mecA gene and remains susceptible to antistaphylococcal penicillins. Nafcillin, oxacillin, or cefazolin are preferred over vancomycin for MSSA because t...

The T-cell receptor is the antigen-recognition molecule on T lymphocytes, a heterodimer of alpha and beta chains that binds peptide presented on MHC. Unlike antibody, it recognizes only processed peptide displayed by MHC, never free anti...

The glycocalyx is a polysaccharide slime layer secreted by bacteria that mediates adherence to surfaces and forms the matrix of biofilm. It is the key virulence factor of Staphylococcus epidermidis on prosthetic devices.

A carbuncle is a deep, coalescing cluster of infected hair follicles forming a single inflammatory mass that drains through multiple openings. It is usually caused by Staphylococcus aureus and typically requires incision and drainage plu...

A furuncle, or boil, is an abscess of a single hair follicle extending into the dermis and subcutaneous tissue, presenting as a tender, fluctuant, erythematous nodule. Staphylococcus aureus is the usual cause and incision and drainage is...

Staphylococcus epidermidis is a coagulase-negative, novobiocin-sensitive gram-positive coccus that is normal skin flora. It infects prosthetic material by forming biofilm and is the leading cause of prosthetic valve endocarditis and cent...

Nikolsky sign is positive when gentle lateral pressure on apparently normal skin causes the epidermis to shear away. It indicates intraepidermal separation and is seen in pemphigus vulgaris, staphylococcal scalded skin syndrome, and toxi...

Transposition is the movement of a DNA segment, a transposon, from one location in the genome to another. In bacteria it is a major mechanism for spreading antibiotic resistance genes between chromosome and plasmid and between species.

The vanA gene cluster confers high-level vancomycin resistance by replacing the D-alanine-D-alanine terminus of peptidoglycan precursors with D-alanine-D-lactate, to which vancomycin binds a thousandfold less avidly. It is plasmid-borne...

Enterococcus faecalis and faecium are group D gram-positive cocci of the normal gut flora that grow in bile and in 6.5% sodium chloride. They cause urinary tract infection, biliary infection, and subacute endocarditis, and are notable fo...

An exanthem is a widespread rash accompanying a systemic illness, usually viral; the corresponding mucosal eruption is an enanthem. The classic childhood exanthems are traditionally numbered one to six.

Panton-Valentine leukocidin is a pore-forming exotoxin of Staphylococcus aureus that lyses neutrophils and macrophages. It is strongly associated with community-acquired MRSA, recurrent skin abscesses, and necrotizing pneumonia in young...

Serotypes are variants within a bacterial or viral species distinguished by the antigens on their surface, defined by their reaction with specific antisera. Serotype diversity explains repeat infections and shapes vaccine design.

Fomites are inanimate objects that carry infectious organisms and transmit them to a new host, such as doorknobs, stethoscopes, toys, towels, and bedding. They are an important route for pathogens that survive drying.

Coxsackie B viruses are non-enveloped, positive-sense single-stranded RNA enteroviruses of the family Picornaviridae. They are the leading viral cause of myocarditis and pericarditis and also cause pleurodynia (Bornholm disease) and asep...

Bornholm disease, or epidemic pleurodynia, is an acute illness caused by coxsackie B virus producing sudden severe paroxysmal chest or upper abdominal pain from inflammation of intercostal and abdominal muscles. It is also called devil's...

Herpangina is an acute febrile illness of young children caused by coxsackie A virus, producing painful vesicles and ulcers on the soft palate, uvula, and tonsillar pillars. The anterior mouth is spared, distinguishing it from herpetic g...

Salmonella enterica serovar Typhi is a motile, non-lactose-fermenting gram-negative rod that produces hydrogen sulfide and possesses a Vi capsular antigen. It causes typhoid fever, has only a human reservoir, and can establish chronic ga...

Typhoid is the systemic febrile illness caused by Salmonella Typhi, characterized by stepwise rising fever, relative bradycardia, abdominal pain, and rose spots. Intestinal haemorrhage and perforation in the third week are its most dange...

Typhoid fever is an enteric fever caused by Salmonella Typhi, acquired faecal-orally from human carriers or contaminated water. It presents with prolonged fever, relative bradycardia, rose spots, and abdominal symptoms rather than promin...

Rabies is a bullet-shaped, enveloped, negative-sense single-stranded RNA rhabdovirus that causes an almost invariably fatal encephalitis. It travels retrograde along peripheral nerves to the central nervous system and produces Negri bodi...

The Salk vaccine is the inactivated poliovirus vaccine (IPV), given by injection, which produces IgG-mediated protection against paralytic disease. It cannot revert to virulence and is the vaccine used in the United States.

Poliomyelitis is infection with poliovirus, a non-enveloped positive-sense RNA enterovirus, which in a minority of cases destroys anterior horn motor neurons and causes asymmetric flaccid paralysis. Sensation is preserved, which is the k...