Okay, let's tackle this question about amenorrhoea in a lactating mother. First, I need to recall the possible causes of amenorrhoea. Amenorrhoea is the absence of menstrual periods, and in lactating mothers, it's often related to hormonal changes.

Lactation, or breastfeeding, is known to affect the menstrual cycle. The key hormone here is prolactin. Prolactin is released by the anterior pituitary gland and is crucial for milk production. High levels of prolactin can inhibit the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus. GnRH is necessary for the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary, which in turn regulate the menstrual cycle. So if GnRH is inhibited, FSH and LH won't be released properly, leading to anovulation and amenorrhoea.

Now, looking at the options, the correct answer should be related to elevated prolactin. Let's consider the other options. If any of the options mention other causes like pregnancy, stress, or thyroid issues, those are possible distractors. But in a lactating mother, the primary cause is the physiological elevation of prolactin. Other options might include conditions like hypogonadism or pituitary tumors, but those are less likely in the context of lactation. Also, if there's an option about low estrogen levels, that's a result of the inhibited GnRH, but the root cause is still the elevated prolactin.

I should make sure that the explanation clearly states that lactational amenorrhoea is due to high prolactin suppressing GnRH, leading to lack of ovulation and menstruation. The other options might include things like pregnancy (but the question specifies lactating mother, implying not pregnant), stress-induced amenorrhoea, or other causes like PCOS. Each of these would need to be addressed as incorrect because they don't fit the context of lactation.

**Core Concept**
Lactational amenorrhoea is primarily caused by elevated prolactin levels, which suppress hypothalamic GnRH secretion. This disrupts the hypothalamic-pituitary-ovarian axis, leading to anovulation and amenorrhoea.

**Why the Correct Answer is Right**
Prolactin, secreted by the anterior pituitary, inhibits GnRH release from the hypothalamus. Reduced GnRH leads to decreased FSH and LH, preventing follicular maturation and ovulation. In lactating mothers, frequent breastfeeding stimulates sustained prolactin secretion, maintaining this suppression and causing amenorrhoea until lactation decreases.

**Why Each Wrong Option is Incorrect**
**Option A:** *Pregnancy* is incorrect because lactation typically occurs after delivery, and amenorrhoea in this context is not due to pregnancy.
**Option B:** *Stress-induced amenorrhoea* involves cortisol-mediated suppression of GnRH, unrelated to lactation physiology.
**Option C:** *Hypogonadotropic hypogonadism* is a chronic condition, not a transient effect of lactation.
**Option D:** *Thyroid dysfunction* causes secondary amenorrhoea via metabolic derangements, not lactation-specific mechanisms

✓ Correct Answer: D. Increased prolactin level