Relationship between Pyorrhea and Teeth:
Pyorrhea is a dental condition characterized by inflammation and infection of the tissues surrounding the teeth, leading to tooth decay and eventual loss. It primarily affects the gums, alveolar bone, and periodontal ligament, causing symptoms such as bleeding gums, bad breath, and tooth mobility.

Identifying the Relationship:
To find the related word/letters/number for "Eczema," we need to understand the relationship between the given words. In this case, the relationship is a specific medical condition and the part of the body it affects.

The word "Pyorrhea" is related to "Teeth" because Pyorrhea is a dental condition that affects the teeth and surrounding structures.

Applying the Relationship:
Now, let's apply the same logic to the given options and find the related word/letters/number for "Eczema."

a) Skin: Eczema is a skin condition characterized by inflammation and irritation of the skin. It causes redness, itching, and rashes on the skin. This option is the correct answer because it follows the same pattern as the given relationship between Pyorrhea and Teeth.

b) Heart: There is no direct relationship between Eczema and the Heart. Eczema does not affect the heart or its surrounding structures.

c) Eye: There is no direct relationship between Eczema and the Eye. Eczema primarily affects the skin, not the eyes.

d) Lungs: There is no direct relationship between Eczema and the Lungs. Eczema is not a respiratory condition and does not affect the lungs.

Conclusion:
Based on the given options, the related word for "Eczema" is "Skin" because Eczema is a skin condition, just as Pyorrhea is a dental condition. Therefore, the correct answer is option 'A' - Skin.