Which of the following causes typhoid fever in human ?

Question

Accepted Answer

The correct option is 2:

The correct answer is Salmonella, specifically the bacterium *Salmonella enterica* serovar Typhi, which is the sole causative agent of typhoid fever in humans. Typhoid fever is a highly contagious, systemic illness acquired through the ingestion of food or water contaminated with the feces of an infected person. Unlike simple gastroenteritis, which other *Salmonella* serovars may cause, *S. typhi* is virulent because it is capable of crossing the intestinal wall and invading the bloodstream, where it then travels to and multiplies in organs such as the spleen, liver, and bone marrow. This invasive nature explains the disease's characteristic symptoms, including a high, sustained fever (often referred to as a step-ladder pattern), severe headache, abdominal pain, and sometimes a temporary rash called "rose spots." Due to its ability to reside in the gallbladder of carriers (like the historical figure "Typhoid Mary"), the bacteria can be shed for long periods, making sanitation and personal hygiene critical for preventing its spread.

The incorrect option is 1:

*Shigella* is the genus of bacteria responsible for causing shigellosis, commonly known as bacillary dysentery. Although it is a Gram-negative bacterium that infects the digestive tract, it is medically and pathologically distinct from typhoid fever. *Shigella* typically remains localized to the colon (large intestine), where it invades the lining of the colon's mucosa to cause severe inflammation and ulcers. The primary and defining symptom of shigellosis is frequent, watery diarrhea that is characteristically bloody and mucoid, often accompanied by fever and abdominal cramps. It does not cause the widespread systemic infection or sustained high fever seen in classic typhoid fever, making it the wrong choice.

The incorrect option is 3:

*Toxoplasma* refers to *Toxoplasma gondii*, which is an obligate intracellular protozoan parasite, not a bacterium. This parasite is the cause of the disease known as toxoplasmosis. Humans typically contract the infection by ingesting oocysts (passed in the feces of infected cats) or by consuming tissue cysts in undercooked meat. For most healthy individuals, toxoplasmosis is either asymptomatic or causes mild, flu-like symptoms, as the immune system keeps the parasite in check. However, it can cause severe or life-threatening illness in immunocompromised people or lead to congenital defects if a woman is infected during pregnancy, illustrating a fundamentally different infectious agent and disease presentation from bacterial typhoid fever.

The incorrect option is 4:

Microfilaria are not causative agents of typhoid fever, but rather the pre-adult larval forms of parasitic nematode (round) worms, which cause the chronic disease filariasis. These worms are multi-cellular organisms (helminths), which are a completely different class of pathogen than the bacteria that cause typhoid. The parasitic worms are transmitted to humans through the bite of infected arthropods, most commonly mosquitoes, which inject the larval stage. Filarial worms are known to cause chronic diseases, most notably lymphatic filariasis (elephantiasis), where adult worms block the lymphatic system, leading to severe, debilitating swelling of limbs or other body parts.