Introducing a man, a girl says, "He is the only son of my mother's mother." How is the man related to the girl?

Introduction / Context:
This blood relation question is straightforward once you decode my mother's mother and then find her only son. The problem asks you to identify how that man is related to the girl introducing him. The key is understanding that my mother's mother is the girl's maternal grandmother, and her only son is therefore the girl's maternal uncle.

Given Data / Assumptions:
- Speaker: a girl. - The man is the only son of my mother's mother. - My mother's mother is the girl's maternal grandmother. - Only son means the grandmother has exactly one male child. - We assume standard terminology for maternal and paternal relatives.

Concept / Approach:
Start with my mother's mother, which is the maternal grandmother. Her only son would necessarily be a brother of the girl's mother, because both are children of the same grandmother. A brother of one's mother is called a maternal uncle. Therefore, this man is the girl's maternal uncle. The presence of both generic uncle and maternal uncle in the options requires you to pick the more specific and correct one.

Step-by-Step Solution:
Step 1: Identify my mother's mother as the girl's maternal grandmother. Step 2: The only son of the girl's maternal grandmother is the grandmother's one male child. Step 3: Since the grandmother has both the girl's mother and this son as children, that son is the mother's brother. Step 4: A brother of one's mother is, by definition, a maternal uncle. Step 5: Therefore, the man being introduced is the girl's maternal uncle.

Verification / Alternative check:
If you draw a short tree, place the grandmother at the top; her children are the girl's mother and the man (her only son). The girl is then a child of the mother. From the girl's point of view, the man is in the same generation as her mother, is male and is her mother's brother. This is precisely the role of a maternal uncle. There is no indication that he is on the father's side, so paternal uncle and father are ruled out.

Why Other Options Are Wrong:
- Uncle (generic) is less specific than maternal uncle; since the question clearly places the man on the mother's side, the correct refined term is maternal uncle. - Paternal Uncle would be a brother of the girl's father, which is not what my mother's mother's only son describes. - Father: The girl's father is not described; the man is explicitly her grandmother's son, not necessarily her parent. - Brother-in-law: This would be the husband of a sibling or the brother of a spouse, which is unrelated to the description given.

Common Pitfalls:
A frequent slip is to stop reading at uncle without noticing whether he is on the mother's or father's side. Another error is to reason that since he is the grandmother's only son, he might be the girl's father, but that would require additional information linking him directly as a parent. The question deliberately traces through the mother to make it clear that he is her brother, not her husband.

Final Answer:
The man is the girl's maternal uncle.