A 6-year-old boy whose family recently immigrated to the United States is brought to the office due to sore throat and fever. Examination shows mild tonsillar erythema with exudates. A sample from the exudates is obtained. On microscopic evaluation, there are clumped, gram-positive bacteria with polar granules that stain deeply with aniline dyes. Which of the following best explains the pathogenicity of the organism most likely responsible for this patient's condition?

Question

A 6-year-old boy whose family recently immigrated to the United States is brought to the office due to sore throat and fever.  Examination shows mild tonsillar erythema with exudates.  A sample from the exudates is obtained.  On microscopic evaluation, there are clumped, gram-positive bacteria with polar granules that stain deeply with aniline dyes.  Which of the following best explains the pathogenicity of the organism most likely responsible for this patient's condition?

Quizplus · Accepted Answer

11ec160b_3820_d5bf_9249_39027e954320_MD0001_00 Corynebacterium diphtheriae are nonmotile, unencapsulated, gram-positive rods. On microscopic examination, they are often found in clumps (classically said to resemble Chinese characters) or joined in V- or Y-shaped chains. Their cytoplasm contains metachromatic granules that stain with aniline dyes (eg, methylene blue). C diphtheriae produce classic 2-subunit AB exotoxin. The B (binding) subunit binds specifically to the heparin-binding epidermal growth factor receptor on cardiac and neural cells, which explains the toxin's affinity for heart and brain tissue. The B subunit induces endocytosis of the toxin, and the subsequently released A (active) subunit inhibits host cell protein synthesis by catalyzing the ADP-ribosylation of protein elongation factor 2 (EF-2). EF-2 is necessary for tRNA to insert new amino acids into the growing protein chain during translation. Therefore, the toxin causes cell death by inhibiting cell protein synthesis. (Choice A) Certain virulent enteric bacteria (eg, Campylobacter jejuni, Vibrio cholerae) release AB exotoxins that activate electrolyte transport. These toxins have 5 B subunits responsible for cell binding and toxin endocytosis and 1 A subunit that stimulates a G protein to increase intracellular cAMP concentrations. As a result, active efflux of sodium and chloride ions occurs, and the subsequent cell water loss into the gut lumen leads to watery diarrhea and dehydration. (Choice B) Botulinum toxin is a neurotoxin that blocks exocytosis of acetylcholine-containing presynaptic vesicles from peripheral nerve terminals, causing cranial and peripheral nerve palsies, muscle weakness, and respiratory paralysis. Tetanus toxin inhibits the release of the inhibitory neurotransmitters gamma-aminobutyric acid and glycine. Classic manifestations of tetanus include muscle spasms and autonomic hyperactivity. (Choice C) Clostridium perfringens releases a cytolytic toxin (alpha-toxin, or phospholipase C) that degrades cell membrane phospholipids, causing cell destruction. (Choice E) Superantigens (eg, staphylococcal and streptococcal toxic shock toxins) bind nonspecifically to T cell receptors and major histocompatibility complex class II molecules on antigen-presenting cells, leading to widespread T cell stimulation and inflammation. However, they are classically implicated in toxic shock syndrome related to tampon use or nasal packing, not upper respiratory infections. Educational objective: Diphtheria exotoxin inhibits host cell protein synthesis by catalyzing the ADP-ribosylation of host cell elongation factor-2.

A 6-Year-Old Boy Whose Family Recently Immigrated to the United