Question

The lung is a more common site for primary tuberculosis than the duodenum because A. the mycobacterium that causes tuberculosis cannot survive in organs other than the lungs. B. the lung has a more readily available supply of oxygen than does the alimentary canal. C. the bile salts present in the small intestine destroy the tubercle bacilli. D. the low \(\mathrm{pH}\) of the digestive secretions in the stomach kills mycobacteria before they enter the duodenum.

Short answer

B. The lung has a more readily available supply of oxygen than does the alimentary canal.

Step-by-step solution

- Understand the question

The question is asking why the lung is a more common site for primary tuberculosis compared to the duodenum.

- Analyze each option

Examine each provided answer choice to determine which one is the most accurate explanation.

- Option A Analysis

Option A states that the mycobacterium that causes tuberculosis cannot survive in organs other than the lungs. This is incorrect because mycobacteria can infect other organs as well.

- Option B Analysis

Option B states that the lung has a more readily available supply of oxygen than does the alimentary canal. Mycobacteria thrive in oxygen-rich environments, thus making this a potential correct answer.

- Option C Analysis

Option C suggests that bile salts present in the small intestine destroy the tubercle bacilli. This is not the primary reason for the lower incidence of tuberculosis in the duodenum.

- Option D Analysis

Option D indicates that the low pH of digestive secretions in the stomach kills mycobacteria before they enter the duodenum. While this is a valid point, it focuses on the stomach rather than the duodenum.

- Select the most accurate option

The most accurate explanation aligns with Option B, which addresses the oxygen supply that mycobacteria need to thrive, making the lungs a more likely site for primary tuberculosis.

Key concepts

Mycobacterium tuberculosis

Mycobacterium tuberculosis is the bacterium responsible for tuberculosis (TB). It is a pathogen that primarily targets the respiratory system, but it can also infect other parts of the body.
This bacterium is transmitted through airborne particles when an infected person coughs, sneezes, or talks. When these droplets are inhaled, the bacteria can settle in the lungs.
Mycobacterium tuberculosis is a slow-growing bacterium and can evade the immune system, making it challenging to treat. It can persist in a dormant state, leading to latent TB, which can reactivate later. The structure of the bacterium includes a waxy cell wall, which makes it resistant to many antibiotics.
The understanding of its characteristics helps us to comprehend why certain environments, such as the lungs, are more conducive to TB infection.

Lung oxygen levels

The lungs provide an optimal environment for Mycobacterium tuberculosis due to their high oxygen levels. Tuberculosis bacteria are obligate aerobes, meaning they require oxygen to grow and survive.
The alveoli in the lungs are richly supplied with oxygen, making them an ideal site for the bacteria to thrive. This is why primary tuberculosis commonly occurs in the lungs rather than other organs.
In contrast, the alimentary canal, including the duodenum, has lower oxygen levels, making it less suitable for mycobacterial growth. The high oxygen availability in the lungs supports the rapid multiplication and persistence of the bacteria.
Furthermore, the lungs’ extensive network of blood vessels facilitates the dispersal of the bacteria, contributing to the spread within the pulmonary system.

Digestive tract defense mechanisms

The human digestive tract has several defense mechanisms that reduce the likelihood of tuberculosis infection. One of these is the presence of bile salts in the small intestine, which can damage or kill many types of bacteria, including Mycobacterium tuberculosis.
Additionally, the stomach produces acidic digestive secretions with a low pH, creating a hostile environment for ingested pathogens. These acidic conditions can destroy Mycobacterium tuberculosis before it even reaches the duodenum.
Together, these factors contribute to the lower incidence of primary tuberculosis infections in the digestive tract compared to the more hospitable environment of the lungs.
These defenses are vital in maintaining the integrity of the digestive system and preventing infections from orally ingested pathogens.

Organ-specific infection

Tuberculosis infections are organ-specific due to the particular needs of Mycobacterium tuberculosis and the environments within different organs. The lungs, with their high oxygen content, provide the best conditions for the bacteria to thrive.
In contrast, other organs, like the duodenum, present less favorable conditions due to lower oxygen levels and defensive mechanisms. However, Mycobacterium tuberculosis is versatile and can infect various organs, although less commonly and usually in cases where the immune system is compromised.
When TB spreads beyond the lungs, it can lead to extrapulmonary tuberculosis, affecting organs such as the kidneys, spine, or brain. This demonstrates the adaptability and potential severity of the infection.
Knowledge of these organ-specific preferences helps in understanding disease progression and in developing targeted treatment strategies.